The atlas codes are available in the atlas dataset itself
3dinfo -atlas_points TTatlas+tlrc
or with
whereami -show_atlas_code
You can extract regions with the whereami command or any other afni command. See the examples in afni11_roi.pdf like this one:
whereami -mask_atlas_region TT_Daemon:left:hippocampus -prefix Lhip
https://afni.nimh.nih.gov/pub/dist/edu/latest/afni11_roi/afni11_roi.pdf