efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of the event flag to be set. The efn argument is a
    longword containing this number; however, $SETEF uses only the
    low-order byte.

    Two local event flag clusters are local to the process: cluster 0
    and cluster 1. Cluster 0 contains event flag numbers 0 to 31, and
    cluster 1 contains event flag numbers 32 to 63.

    There are two common event flag clusters: cluster 2 and cluster
    3. Cluster 2 contains event flag numbers 64 to 95, and cluster 3
    contains event flag numbers 96 to 127.