SRX is a second run group class for all sports racers (B,C,D,E & F).
FBX is the same as SCCA class FB 1000 and runs under the current year/month SCCA rules.
These are provisional classes which have not achieved Championship class status. Once the entry numbers are high enough to become a Championship class the rule set will be added to the Competition Regulations. See rule 1301. A. for details.
Since the majority of the clubs have decided to combine groups 3 & 6 due to diminishing car counts, I think it is time to remove SRX as a class. It never made the minimum to be considered as a championship class.