efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of any event flag within the event flag cluster to be
    used. The efn argument is a longword containing this number;
    however, $WFLAND uses only the low-order byte. Specifying the
    number of an event flag within the cluster serves to identify the
    event flag cluster.

    There are two local event flag clusters: 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.

 mask

    OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Event flags for which the process is to wait. The mask argument
    is a longword bit vector wherein a bit, when set, selects the
    corresponding event flag for which to wait.