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They represent the TCP flags, indeed. RFC 793, 3.1:

Control Bits: 6 bits (from left to right):

URG: Urgent Pointer field significant
ACK: Acknowledgment field significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender

And additions:

NS: ECN-nonce - concealment protection. RFC 3540
CWR: Congestion window reduced. RFC 3168
ECE: ECN-Echo. RFC 3168

From this we can deduce:

TCP:RA = RST, ACK
TCP:FA = FIN, ACK
TCP:PA = PSH, ACK
TCP:S = SYN
TCP:SEC = SYN, ECE, CWR