From Team Fortress Wiki
The Mission Briefing is an informative page in the style of a chalkboard that is displayed to the player after connecting to a game. Its purpose is to provide useful details to players about the current map and game type like the map name, game mode, any objectives that must be completed in order to win, and any additional notes relating to the match at hand. Images are provided alongside the text to give additional visual information of key locations of the map, e.g. an important thoroughfare or control point. The Mission Briefing is automatically determined by the current game mode in the same way that the game mode is determined and displayed on the loading screen. The Mission Briefing may also be followed by an instructional video that is designed to showcase the objective for both teams, and how said objective must be completed. The video may be skipped.
Modifying the Mission Briefing
Custom maps can override the Mission Briefing text and images.
Modifying the text
The objective and notes text is stored in a localized text file for the map, and is located in the
tf/maps/ folder. The text file is named after the map's full filename (without the extension), while non-English versions may be created by appending the language name.
mapname.bsp mapname_danish.txt mapname_dutch.txt mapname_english.txt mapname_finnish.txt mapname_french.txt mapname_german.txt mapname_italian.txt mapname_japanese.txt mapname_korean.txt mapname_norwegian.txt mapname_polish.txt mapname_portuguese.txt mapname_russian.txt mapname_schinese.txt mapname_spanish.txt mapname_swedish.txt mapname_tchinese.txt
cp_dustbowl_b1 would have required the file
Note: In most cases, the automatically displayed Mission Briefing text will work. This technique should only be used for maps with custom game modes or if additional information may be required.