Under some circumstances, a full file specification may contain
    more characters than the 255 bytes allowed by unmodified
    applications. If a file specification that such an application
    needs exceeds 255 bytes in length, RMS generates a shorter
    file specification by abbreviating the directory to a DID
    abbreviation, and if necessary, the filename to a FID
    abbreviation.

    When the file specification is too long, RMS first attempts to
    generate a shorter directory specification by identifying the
    directory with its directory ID. This shorter specification is
    referred to as a DID abbreviation.

    TEST$ODS5:[5953,9,0]Alghero.TXT;1

    Note that this form of the directory name must have three numbers
    and two commas to avoid ambiguity with UIC format directory
    names. With the DIRECTORY command you can view the shorter
    DID abbreviation version as well as the full version of a file
    specification.