The AFNI_atlas_spaces.niml file is a simple text file that specifies almost everything AFNI knows about atlases, spaces and transformations. An atlas can have a name that is different than its filename, and you can see this with "whereami -show_atlases" which shows the names and dataset file names for all atlases. The left/right business is used where the atlas (like the Talairach daemon atlas) uses the same name on the left and right; otherwise, just include the left/right in the region name.