What does containment and mop up of a wildfire mean?

Firefighters have achieved considerable progress in their work to contain the wildfires that have been raging throughout Southern California over the past week.
Although the fight against the Thomas fire in Ventura County continues to be a titanic effort — the 173,000-acre blaze was only 15 percent contained as of Sunday midday — firefighters have reported significantly better containment percentages for other fires.
This big board is posted with information and a fres map every day so that a variety of agencies can come together for “co-operators meetiings” during the day. Firefighters, first responders and support personnel are using the Hansen Dam Recreation Area as a staging area and command post for the Creek fire. Lakeview Terrace, CA 12/8/2017 (Photo by John McCoy, Los Angeles Daily News/SCNG)U.S. Forest fire crews fight fire with fire as they set off huge backfires to cut off the northern flank of the Thomas fire near Rose Valley recreation area on Saturday. Dec. 9, 2017 in the Los Padres National Forest. The Thomas Fire has spread to near 150,000 acres. (Photo by Gene Blevins for the Los Angeles DailyNews/SCNG)U.S. Forest fire crews fight fire with fire as they set off huge backfires to cut off the northern flank of the Thomas fire near Rose Valley recreation area on Saturday. Dec. 9, 2017 in the Los Padres National Forest. The Thomas Fire has spread to near 150,000 acres. (Photo by Gene Blevins for the Los Angeles DailyNews/SCNG)U.S. Forest fire crews fight fire with fire as they set off huge backfires to cut off the northern flank of the Thomas fire near Rose Valley recreation area on Saturday. Dec. 9, 2017 in the Los Padres National Forest. The Thomas Fire has spread to near 150,000 acres. (Photo by Gene Blevins for the Los Angeles DailyNews/SCNG)U.S. Forest fire crews fight fire with fire as they set off huge backfires to cut off the northern flank of the Thomas fire near Rose Valley recreation area on Saturday. Dec. 9, 2017 in the Los Padres National Forest. The Thomas Fire has spread to near 150,000 acres. (Photo by Gene Blevins for the Los Angeles DailyNews/SCNG)U.S. Forest fire crews fight fire with fire as they set off huge backfires to cut off the northern flank of the Thomas fire near Rose Valley recreation area on Saturday. Dec. 9, 2017 in the Los Padres National Forest. The Thomas Fire has spread to near 150,000 acres. (Photo by Gene Blevins for the Los Angeles DailyNews/SCNG)Los Angeles County firefighters from station 28 attack a fire that broke out Wednesday morning at the Wildlife Waystation at the Creek fire which has burned more than 11,000 acres and destroyed at least 30 structures. ( Photo by David Crane, Los Angeles Daily News/SCNG)A firefighter from the County of Los Angeles puts water on hotspots that may have been burning in this house on Via San Diego street in the Santiago Estates housing tract in Sylmar on Thursday, Dec. 7, 2017. (Photo by John McCoy, Los Angeles Daily News/SCNG)A firefighter from the County of Los Angeles puts water on hotspots that may have been burning in this house on Via San Diego street in the Santiago Estates housing tract in Sylmar, CA 12/7/2017 (Photo by John McCoy, Los Angeles Daily News/SCNG)Los Angeles County firefighters from station 28 attack a fire that broke out Wednesday morning at the Wildlife Waystation at the Creek fire which has burned more than 11,000 acres and destroyed at least 30 structures. ( Photo by David Crane, Los Angeles Daily News/SCNG)Firefighters battle the Rye Fire along Newell Ranch Road in Santa Clarita on Tuesday, Dec. 05, 2017. (Photo by Ed Crisostomo, Los Angeles Daily News/SCNG)Firefighters battle the Rye Fire along Newell Ranch Road in Santa Clarita on Tuesday, Dec. 05, 2017. (Photo by Ed Crisostomo, Los Angeles Daily News/SCNG)Firefighters battle the Rye Fire from the air and the ground near Six Flags Magic Mountain in Santa Clarita on Tuesday, Dec. 05, 2017. (Photo by Ed Crisostomo, Los Angeles Daily News/SCNG)A Super Scooper aircraft makes a water drop on a brushfire fanned by heavy winds burning in the hills near Peter J. Pitchess Detention Center in Castaic on Tuesday, December 5th, 2017. (Photo by Dan Watson / SCNG)A view of the area destroyed by the Skirball Fire in Bel-Air on Thursday, Dec. 07, 2017. (Photo by Ed Crisostomo, Los Angeles Daily News/SCNG)Los Angeles City Fire Dept., firefighters clear hot spots at a home destroyed by the Skirball Fire on Casiano Road in Bel-Air on Thursday, Dec. 07, 2017. (Photo by Ed Crisostomo, Los Angeles Daily News/SCNG)LA City firefighters put out hot spots in the Skirball Fire on a canyon along Linda Flora Drive in the Bel Air neighborhood of Los Angeles, California, on Wednesday, December 6, 2017. (Photo by Jeff Gritchen, Orange County Register/SCNG)LA City firefighters put out hot spots in the Skirball Fire on a canyon along Linda Flora Drive in the Bel Air neighborhood of Los Angeles, California, on Wednesday, December 6, 2017. (Photo by Jeff Gritchen, Orange County Register/SCNG)A firefighter calls in a water drop as he fights the Liberty Fire near Los Alamos in Murrieta Thursday, December 7, 2017. (Andrew Foulk for The Press Enterprise/SCNG)Firefighters walk a fireline as they battle the Liberty Fire burns near Los Alamos in Murrieta Thursday, December 7, 2017. (Andrew Foulk for The Press Enterprise/SCNG)People take photos as a DC-10 makes a Phos-Chek, or fire retardant, drop on the Liberty Fire burning near Los Alamos in Murrieta Thursday, December 7, 2017. (Andrew Foulk for The Press Enterprise/SCNG)Firefighters work on hotspots from Lilac Fire that destroyed homes in Bonsall Friday, December 8, 2017. FRANK BELLINO, THE PRESS-ENTERPRISE/SCNGA firefighter works on hotspots on the destroyed homes at Rancho Monserate Country Club from Lilac Fire near Interstate 15 Friday, December 8, 2017. FRANK BELLINO, THE PRESS-ENTERPRISE/SCNGFirefighters work on hotspots from the Lilac Fire that destroyed homes in Bonsall Friday, December 8, 2017. FRANK BELLINO, THE PRESS-ENTERPRISE/SCNGFirefighters battle the fast-moving Lilacfire south of Fallbrook near Bonsall along Interstate 15 and state Route 76 in Bonsall Thursday, December 7, 2017. FRANK BELLINO, THE PRESS-ENTERPRISE/SCNGShow Caption of Expand
Firefighters assigned to Los Angeles County blazes have achieved 90 percent containment of both the Creek and Rye fires, which have respectively burned in the Sylmar and Santa Clarita areas since Tuesday. And in Riverside County, where the Liberty fire flared up Thursday in Murrieta, fire crews accomplished 100 percent against that blaze as of Saturday night.
So, what does “containment” mean? The Los Angeles Fire Department’s @LAFDtalk Twitter account provided an answer Sunday morning when replying to another Twitter user’s question as to why the Skirball fire in the Bel-Air area was not fully contained despite an apparent absence of flames. (The question referred to that fire as being 50 percent contained on Saturday afternoon. By Sunday afternoon, firefighters had achieved 75 percent containment.)

@LAFDtalk hey there, i was curious- fires that appear not to have any active flames (like skirball) – how come it’s only considered 50% contained?
— Jennifer (@gillovnot) December 9, 2017
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In the fire service, ‘containment’ has specific meaning – a fire break has been constructed AROUND the fire perimeter This can include any combination of manually created break, hose lines, natural barriers Containment may/may not happen while still visible flames https://t.co/ZnI6cvU3ja
— LAFD Talk (@LAFDtalk) December 10, 2017
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Essentially, containment is another word for “surrounded.” Whereas firefighters attacking a structure fire with the likes of hoses and axes will declare a blaze to be “knocked down” after they gain control of the flames, firefighters employing heavy machinery and hand tools to clear vegetation — and thus to deny fuel from a wildfire — use the word containment to describe how much progress they have made hacking and digging a perimeter around a blaze.
“We actually have to put a line all the way around the fire,” Angeles National Forest spokesman Nathan Judy said. Building that line can be accomplished by bulldozers or ground crews with hand tools.
Firefighters can also increase containment percentage by setting up hose lines near a wildfire, Los Angeles Fire Department spokeswoman Margaret Stewart said.
The containment percentage, however, does not necessarily correlate to safety level around the fire zone, she said. That’s because it’s possible for winds to carry burning embers to the other side of a fire break.

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The National Weather Service predicted dangerous conditions — low humidity combined with gusty Santa Ana winds — to continue to be a risk factor on Sunday. A red flag warning is in effect through 8 p.m. for much of Southern California.
Firefighters continue “mopping up” activities after achieving a significant containment percentage. That job requires firefighters to venture some 100 to 300 feet into burned areas to attack any remaining hot spots, Judy said.
Tackling remaining hot spots makes it possible for firefighters to declare a wild fire is “controlled,” according to the National Wildfire Coordinating Group.
Here’s the NWCG’s definitions for the terms “contained,” “controlled” and “out.”

Contained: When firefighters say a fire is “contained” a control line has been completed around the fire which can reasonably be expected to stop the fire’s spread.
Controlled: For a fire to be called “controlled,” firefighters need to have removed any unburnt fuel and cooled down all hot spots adjacent to control lines to the point that they can reasonably be expected to hold.
Out: A fire is considered “out” when no hot spots are detected within containment lines
for at least 48 hours.

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Source: Oc Register