PUMS variable "meanings"

Hello. Does anyone know of any good documentation about the meaning of each of the PUMS variables? The data dictionaries only give the "full name" of the variables, not necessarily what they are. I recall seeing such a document at some point, but can't find it now. In particular, I'm trying to figure out how the variables "OC" ("Own Child"), "PAOC" (Presence and Age of Own Child), and "RC" ("Related Child") are the same/different.

Any help would be appreciated!