VMS Help Sys Parameters, RMS_DFNBC  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    RMS_DFNBC specifies a default block count for network access to
    remote, sequential, indexed sequential, and relative files.

    The network block count value represents the number of blocks
    that RMS is prepared to allocate for the I/O buffers used to
    transmit and receive data. The buffer size used for remote file
    access, however, is the result of a negotiation between RMS and
    the remote file access listener (FAL). The buffer size chosen is
    the smaller of the two sizes presented.

    Thus, RMS_DFNBC places an upper limit on the network buffer size
    that is used. It also places an upper limit on the largest record
    that can be transferred to or from a remote file. In other words,
    the largest record that can be transferred must be less than or
    equal to RMS_DFNBC multiplied by 512 bytes.

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

    RMS_DFNBC is an AUTOGEN and DYNAMIC parameter.