From Populous Wiki
Mappacks are a selection of new levels made by fans of the games. They are mostly multiplayer levels for matchmakers. Any fan can design levels with an editor and create levels for you and your friends to play on. Mappacks can come in a variety of formats, with some including an installer to replace the game files, and others being a simple archive of the maps.
Mappacks on Populous Reincarnated
All mappacks designed for the Populous Reincarnated Matchmaker are stored in a standard zip file and contain an extra file named mappack.txt. The mappack.txt file contains all the information about the pack and maps including names, pack versions, creators, and the file names for each map's .dat and .hdr file. This allows the files in the pack to have any name without the need to conform to the level2XXX format used by Populous.
The mappack.txt file also contains an 8 character signature at the top of the file that must be generated by a Populous Reincarnated Mappack Checker before the pack can be used in the Populous Reincarnated Matchmaker. This ensures that all mappacks released have been reviewed to eliminate many bugs and meet a quality standard, and also means that the mappack has been added to the PopRe mappack database, meaning it can be downloaded in the matchmaker by all players.