RMS_DFMBC specifies a default multiblock count only for record
    I/O operations, where count is the number of blocks to be
    allocated for each I/O buffer.

    You can set this system parameter with the DCL command SET RMS_
    DEFAULT/SYSTEM and display the parameter with the SHOW RMS_
    DEFAULT command.

    RMS_DFMBC is an AUTOGEN and DYNAMIC parameter.