If this is from the abrelax application, then I believe the “S” stands for “Success”, “F” is for failure of filters, “C” would be failure of loop closure, “X” would be failure of both loop closure and filters, and “P” would be for some other problem in the abrelax protocol.
Filters here would be the radius of gyration, contact order, and sheet filters, as well as other experimental evidence based filters (PDDF, SAXS), if applicable.