Property:Person-mlbamID
From SABR Encyclopedia
This is a 6-digit ID assigned by MLBAM, mostly to Major Leaguers and recent minor leaguers. Despite being numeric, we treat these as being of type string.
This ID need not be unique; there is evidence the same person has been issued multiple IDs by MLBAM in the past. However, it generally appears that only one ID per person is in common (public) use.