This rule option refers to the TCP sequence number.
Essentially, it detects if the packet has a static sequence number set, and is therefore pretty much unused. It was included for the sake of completeness.
The above comment notwithstanding, the check is used in a number of places:
seq: 101058054 seq: 3868 seq: 6060842 seq: 674711609 seq:0 seq:1958810375
Definition at line 950 of file rlp.h.