Alternate locations
From Proteopedia
Some PDB files have alternate locations (altlocs) for some atoms. Each atom that has altlocs is marked with an altloc identifier, typically A and B.
OCA highlights the structure with the most 'altlocs', see within Proteopedia for information mined from OCA.
Note
It looks like in the mmCIF format correspondence may be 'alt_id'. However, because that is listed as corresponding to 'alt_loc' , it is unclear if that is the same as 'altLoc' listed in the PDB format guide.