The MEMORY_RESERVE_31BIT MUF Startup Option is explained on the documentation webpage "Administrating > CA Datacom/DB Database and System Administration > Using the Multi-User Facility > Modifying MUF Startup Options"
This option, provided in version 15.1 of CA Datacom/DB, allows the MUF and the operating system to run processes when MUF memory is insufficient to complete them. Without this option, processes needing more memory than is available to the MUF would typically cause the MUF to fail with a memory-related abend, such as S878.
There are two values supplied to this option: the first value (0 - 99M, with a default of 2M) is reserved by the MUF for IBM/Operating System processes; the second value (0 - 99M, with a default of 1M) is reserved for CA Datacom critical functions (such as issuing a database close).
As explained in the documentation:
Use this option to reserve memory for both IBM and Datacom critical processes. Before attempting to allocate storage from the operating system, the specified thresholds are checked. If the thresholds were reached, the storage is not allocated. The request fails with a DB return code 89(005).
Using this option allows both z/OS and Datacom/DB to gracefully handle out of storage conditions, which in turn can prevent a failure of the Multi-User. Normal processes can continue to the best of its ability once memory is exhausted. For example, without this option it is possible for MUF to fail without proper documentation since z/OS might need storage to document the problem.
This option is set in the MUF Startup Options, and can be adjusted using MUF console commands, the SQL_CONSOLE table, or the DBUTLTY COMM console function.