VMS Help DEFINE, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 logical-name

    Specifies the logical name string, which is a character string
    containing from 1 to 255 characters. The following rules apply:

    o  If the logical name is to be entered into the process or
       system directory logical name tables (LNM$PROCESS_DIRECTORY,
       LNM$SYSTEM_DIRECTORY), then the name can only have from 1 to
       31 alphanumeric characters, including the dollar sign ($)  and
       underscore (_).

    o  If you specify a colon (:)  at the end of a logical name, the
       DEFINE command saves the colon as part of the logical name.
       (This is in contrast to the ASSIGN command, which removes the
       colon before placing the name in a logical name table.) By
       default, the logical name is placed in the process logical
       name table.

    o  If the string contains any characters other than uppercase
       alphanumerics, the dollar sign, or the underscore character,
       enclose the string in quotation marks (" ").  Use two sets of
       quotation marks ("" "")  to denote actual quotation marks.
       When you enclose a name in quotation marks, the case of
       alphabetic characters is preserved.

 equivalence-name[,...]

    Specifies a character string containing from 1 to 255 characters.
    The following rules apply:

    o  If the string contains any characters other than uppercase
       alphanumerics, the dollar sign, or the underscore character,
       enclose the string in quotation marks. Use two sets of
       quotation marks to denote an actual quotation mark. Specifying
       more than one equivalence name for a logical name creates a
       search list.

    o  When you specify an equivalence name that will be used as a
       file specification, you must include the punctuation marks
       (colons, brackets, periods) that would be required if the
       equivalence name were used directly as a file specification.
       Therefore, if you specify a device name as an equivalence
       name, you must terminate the equivalence name with a colon.

    The DEFINE command allows you to assign multiple equivalence
    names to a single logical name. For example, you can use the same
    logical name to access different directories on different disks
    or to access different files in different directories.