jelos.org/docs/systems/build-engine.md

Build Engine

.build files

These files must be created for each build engine game that will be launched with raze. The file contains a PATH variable and an optional GRP variable. The PATH variable points to the subfolder containing the game's GRP file. The optional GRP variable is used to identify the specific GRP file to load for games with multiple GRP files.

Example: /roms/build/shadow warrior.build contains

PATH=sw
GRP=SW.GRP
-- end --

where the Shadow Warrior games files are stored in subfolder sw, i.e. /roms/build/sw/

Note: don't leave any space between GRP or PATH and = and enclose filenames containing spaces with "quotes"

Note: add -- end -- to the end of the file if it contains multiple lines