The sysplex couple data set retains information about systems that are currently active in the sysplex and information about systems that were previously active but are now inactive. Beginning with z/OS V1R9, inactive system information is retained even across a sysplex-wide IPL, for the benefit of systems management software that requires a historical view spanning multiple instances of the sysplex.

If there are no matching, empty, or reclaimable slots, the incoming system cannot join the sysplex. In this case, the MAXSYSTEM value is too small.

This slot selection algorithm can cause an incoming system to be assigned a higher slot number than expected. For instance, with two systems already active in the sysplex, the next system to IPL might receive slot number 5 rather than 3. Changing the name of a system is one way this can occur, since the sysplex couple data set will retain a slot for the old name. If the MAXSYSTEM value has been chosen to allow for eventual sysplex growth, the slot number assigned can be greater than the maximum number of systems you expect to be active in the sysplex at its current usage.

If a system is assigned a slot number greater than the MAXSYSTEM value with which one of the function couple data sets was formatted, the incoming system will not be able to use that function. For example, suppose the sysplex couple data set is formatted with MAXSYSTEM=16, but the CFRM couple data set is formatted with MAXSYSTEM=12. If a system IPLing into the sysplex is assigned slot number 13, it cannot use the CFRM function, even if there are 12 or fewer systems actually active in the sysplex. For this reason, it is important that all function couple data sets be formatted with the same MAXSYSTEM value used to format the sysplex couple data set.