Library /sys$common/syshlp/pca$help.hlb SET, SOURCE, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 dirname

    Specifies a directory to search. If multiple dirname parameters
    are specified, the specified directories are searched for source
    files in that order. You may specify a OpenVMS logical name for a
    dirname.

    Note that dirname may consist of one, several, or all the fields
    in a full file specification, though it typically consists only
    of a directory name. A full file specification has the following
    format:

    node::device:[directory]file-name.file-type;version-number

    When specifying any of these fields, you must include the
    punctuation for that field as shown in this format.