SEND →     RPL:     ACB field → ACB
                    ARG field = CID of session
                    Note: ECB and EXIT are mutually exclusive.
                    If ECB is used, only one is allowed.
                    ECB field → external ECB
                    ECB field = internal ECB
                    EXIT field → RPL exit routine
                    BRANCH field = YES|NO
                    POST field = SCHED|RESP
                    AREA field → data to be sent or buffer list
                    RECLEN field = length of data to be sent or length
                         of buffer list
                    RESPOND field = (EX|NEX,FME|NFME,RRN|
                         NRRN,QRESP|NQRESP)
                    RTYPE field = (DFSYN|NDFSYN,DFASY|NDFASY,
                         RESP|NRESP)
                    CONTROL field = DATA|BID|BIS|CANCEL|
                         CHASE|LUS|QC|RTR|QEC|
                         RELQ|RSHUTD|SBI|SHUTC|
                         SHUTD|SIGNAL
                    SIGDATA field = signal data (for STYPE=REQ and
                         CONTROL=SIGNAL)
                    CHNGDIR field = (CMD|NCMD,REQ|NREQ)
                    BRACKET field = (BB|NBB,EB|NEB,CEB|NCEB)
                    CHAIN field = FIRST|MIDDLE|LAST|ONLY
                    CRYPT field =YES|NO
                    CODESEL field = STANDARD|ALT
                    STYPE field = REQ|RESP
 
                    If STYPE = RESP,
                         SEQNO field = sequence number
 
                    If STYPE = RESP and RESPOND = EX
                         SSENSEO field = CPM|STATE|FI|RR|0
                         SSENSMO field = system-sense modifier field
                         USENSEO field = user-sense value
 
                    OPTCD field = (SYN|ASY,
                         FMHDR|NFMHDR,
                         CA|CS,
                         LMPEO|NLMPEO,
                         CONTCHN|NCONTCHN,
                         BUFFLST|NBUFFLST,
                         USERRH|NUSERRH,
                         RSPQUED|NRSPQUED)
 
     Register 0 and 15 = return codes
 
     RPL:        RTNCD field = recovery action return code
                 FDB2 field = specific error return code
                 REQ field = request code (34)
                 USER field = data from USERFLD in NIB used
                                         to establish session
                 SEQNO field = sequence number (last sequence
                               number for LMPEO)
                 OBSQVAL field = first sequence number for LMPEO
 
                 If POST = RESP (and it was not overridden)
                      CONTROL field = SNA request code returned
                      on response
                      RESPOND field = (EX|NEX|FME|NFME,RRN|NRRN)
                      CHNGDIR field = REQ indicator returned on
                      response
                      SSENSMI field = CPM|STATE|GI|RR|PATH|0
                      USENSEI field = user-sense value (or 0)