If the CFRM policy specifies the SCMMAXSIZE keyword to indicate that a structure is eligible to use storage-class memory, a lack of storage-class memory might affect structure allocation in a coupling facility at or above CFLEVEL 19.

• The maximum amount of storage-class memory that a structure can use is the smaller of the CFRM policy SCMMAXSIZE value and the total amount of storage-class memory that is configured to the coupling facility. Other factors might further limit the amount.
• The use of storage-class memory increases the amount of control storage that is required to support a given number of entries and elements. It therefore reduces the number of entries and elements that can be accommodated in coupling facility real storage by a structure of a specified size. If the amount of storage-class memory that is configured to the coupling facility is less than the SCMMAXSIZE specification and the structure is therefore allocated to support a smaller maximum amount of storage-class memory than intended by the CFRM policy, it would be capable of containing more entries and elements than it would if allocated in a coupling facility with more configured storage-class memory. That would cause a problem if the structure were to be subsequently rebuilt into a coupling facility with more storage-class memory, because the rebuild new structure instance would not be able to accommodate the same object counts as when the structure was originally allocated. To prevent this conflict, the system limits the number of entries and elements when allocating in a coupling facility with less storage-class memory than specified by the policy.
  Note: This behavior depends on the amount of storage-class memory that is configured to the coupling facility, not the amount of free storage-class memory.
• Storage-class memory is not allocated to the structure until required for use. It is therefore possible to overcommit the storage-class memory that is configured to the coupling facility. It is possible to define the CFRM policy such that the sum of the SCMMAXSIZE values for allocated structures exceeds the total amount of storage-class memory that is available to the coupling facility. To ensure that you have the desired amount of storage available in the event of an application failure that causes the structure to fill up, do not overcommit storage class memory.

The use of storage-class memory affects the structure's resource usage in a complex way. See z/OS MVS Setting Up a Sysplex for additional discussion about its implications.