This is a number, initialized to 1, that increments each time a transmitted #FNF_RPC has been acknowledged. When it reaches 255 and increments, it wraps around to 1. For all other Function Codes, the number is zero.
The receiving SocketNode acknowledges, but does not action, an RPC message that was a duplicate of the previous one.
This is designed to prevent SocketNode taking replicate action if the same RPC is re-transmitted, as part of a retry strategy. In the situation where the remote workstation sees the RPC message transmission and acknowledges it, but the transmitting workstation fails to receive the acknowledgement, a successful re-transmission of the same RPC message will be acknowledged, but a second invocation of the RPC subroutine will not occur. This could happen because SocketNode will tolerate a high degree of inter-workstation communication link disturbance before session termination occurs.
This approach also requires that if the head of the transmission FIFO is discarded, due to session loss, the Sequence Number must be incremented so that the next RPC message sent will not appear as a replicated message.