Required space

OS/390® requires eight list headers to manage a coupling facility list structure for signaling.
So that at least one signaling path can be established, a list structure must be allocatable with at least one list header more than the number needed by MVS™ (#XCF_lists).
To allow the maximum size signal supported by the MVS signaling service (61K bytes) to be inserted into a single list entry, a list structure used for signaling must be allocated so that a list entry can contain up to 64K bytes of data (up to sixteen 4096 byte data elements per data entry).
Therefore, in z/OS, a coupling facility list structure defined for signaling must have a minimum of twenty-five list entries. If this absolute minimum of 25 list entries is not available, the list structure remains defined to MVS but is considered inoperative and cannot be used for signaling.