mil_branch 
Datums for military branches and ranks
Map datums can optionally specify a list of /datum/mil_branch paths. These paths are used to initialize ranks for the map, which contains a list of branch objects used on the map. Each branch definition specifies a list of /datum/mil_rank paths, which are ranks available to that branch.
Which branches and ranks can be selected for spawning is specifed in global.using_map and each branch datum definition, respectively.
A single military branch, such as Fleet or Marines