According to its description, CITY is consitent with the census definition for place. But is there a variable in the ACS micro data that is consistent with census definition for principal city?
The variable METRO provides information on whether a household lived in a metropolitan area. If the household was in a metropolitan area, it was further classified as either within the metropolitan area’s central/principal city or within the remainder of the metropolitan area. I recommend reading the comparability section for METRO to understand the possible limitations of this variable.
I hope this helps.