NetView SSI design change in NetView V6.1,

Design change in z/NetView 6.1

First of all, the reason MSGIFAC was retired is not that the default is SSIEXT, but that there is no longer a choice. Unsolicited MVS message routing is done differently in V6R1 than in any previous release, using any MSGIFAC. Its behavior is closest to SSIEXT, but there are major differences.
With MSGIFAC of SSIEXT in V5R4 and before, the SSI started procedure defines SSI message and command exits to MVS, so any time a WTO, WTOR, or MGCRE is issued, that exit routine gets control. The routine determines if the message/command should be routed to the NetView address space, and if so, allocates a buffer out of the pool reserved for messages (MBUF) or commands (CBUF) and posts the NetView address space indicating something is waiting for it. The NetView address space (CNMCSSIR) finds the earliest queued buffer, copies the contents to the NetView address space, and makes the buffer available again.
In V6R1, the subsystem initialization routine (INITRTN) that is specified on the SUBSYS statement (DSI4LSIT) in an IEFSSNxx PARMLIB member gets control at IPL time. This routine does several things, including creating the Canzlog data space and defining the message and command SSI exits to MVS. (The SSI started task no longer does this.) So, any time a WTO, WTOR, or MGCRE is issued, that exit routine is given control. For messages, with very few exceptions, the routine creates an entry in the Canzlog data space for it. It then determines which NetView address space(s) should be given a chance to automate the message. It sets appropriate flags in the Canzlog entry and posts each respective
NetView address space, notifying it that a message can be processed. The NetView address space (CNMCSSIR, again), finds the oldest entry in Canzlog that needs to be automated and creates an AIFR out of it and automates it. The flow is almost identical for commands containing a NetView designator character.

So in summary, the SSI message buffer pool (MBUF) and command buffer pool (CBUF) were effectively replaced by the Canzlog data space.