SHA256 is used in .GAME as the checksum of choice. It covers file contents to ensure validity. The header is matched against known database included with RC. Basically by checksumming headers a match can be found between what is on disk and the official headers, quickly and easily.
A .GAME file can be broken down into 3 segments, you have NAME headers, ROM headers and SYSTEM headers. Every .GAME file will consist of one or more NAME+SYSTEM headers, and usually at least one ROM header. ROMs are attached to NAMEs. NAME headers are a name for the game and also contain some common features for every game, such as playtype, players, year released, etc. It also includes an english and native name (UTF-8). System headers are specific to each system and are designed to support all current and future software for that system.
Eventually I will release some source and documentation on the format. As it is currently no other emulator author has been interested so I haven't been interested in releasing it all yet.