source-string

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

    Source string to be searched by LIB$INDEX. The source-string
    argument is the address of a descriptor pointing to this source
    string.

 sub-string

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

    Substring to be found. The sub-string argument is the address of
    a descriptor pointing to this substring.