f-ranks are those pomps which have been removed from the universe by the sleeping mama, the ss-rank who embodies and governs the entire pomp reality
incidentally, the details of each rank:
S-rank: extremely old, extremely powerful recurring characters who represent and control one aspect of reality
A-rank: very strong recurring characters
B-rank: slightly powerful recurring characters
C-rank: recurring characters with so little power they are only recurring characters due to having names
D-rank: recurring characters who do not fight, classified as D-rank regardless of their power level
F-rank: failed recurring characters, banned from the universe or from existence