source-string

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

    String that STR$FIND_FIRST_IN_SET compares to the set of
    characters, looking for the first match. The source-string
    argument is the address of a descriptor pointing to the character
    string.

 set-of-characters

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

    Set of characters that STR$FIND_FIRST_IN_SET is searching for
    in the string. The source-string argument is the address of a
    descriptor pointing to the set of characters.