I'm looking for a list of geographic variant codes

Hello colleagues, I'm looking for a list of geographic variant codes. These are found in the 4th and 5th digits of the standard census geoid.

I'm not sure whether ACS actually uses these (other than "00"), but they are fairly common in the new DHC data. The DHC documentation only says that geographic variant code "Indicates the version of a geographic entity, based on the date of the entity’s boundaries." Not helpful without a key.

From context, I can surmise that variant "C2" refers to urban/rural designation, since C2 geographies always have geographic components of 01 (urban) or 43 (rural). Other variant codes include 16, L6, M6, U6, and Z2. There may be others I'm not aware of.

Thanks!