With the filter criteria, "party member," it appears that it is possible to have this value be null. In other words, if I run a filter for "is not party member" or "is party member" the total is significantly different that the total number of people in my database.
In order to isolate the profiles that are not party members, this discrepancy makes it necessary to create a list of people who are party members and then run a filter for profiles that are not on that list.
Thank you Lauren! This isn't something we can prioritize changing but I will ask our writing team to clarify the documentation.