source-string

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    String that STR$FIND_FIRST_NOT_IN_SET searches. The source-string
    argument is the address of a descriptor pointing to the string.

 set-of-characters

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    The set of characters that STR$FIND_FIRST_NOT_IN_SET compares
    to the string, looking for a nonmatch. The set-of-characters
    argument is the address of a descriptor pointing to this set of
    characters.