What are the N/A values for FTOTINC?

There are many N/A values for the variable FTOTINC (family total income). I couldn’t find out who are these N/A values by checking questionnaire because there is no question for family income. Is FTOTINC calculated based upon summing up all personal income in a household? And to which group of people this variable is not applicable? Thanks.

FTOTINC is the sum of personal income (INCTOT) for each member of a family unit (FAMUNIT). Multiple family units can reside in a single household. There are two separate scenarios in the data where a respondent receives an N/A value for FTOTINC. First, if the respondent lives in group quarters (GQ=3 or 4). Second, if the respondent is living in a household (GQ not equal to 3 or 4), under the age of 15 (AGE<15), and not related to anyone else in the household aged 15+ (in practice, FAMSIZE=1).

Hope this helps.