directory-spec

    Specifies any part of an OpenVMS file specification (typically
    a device/directory) that the debugger is to use by default
    when searching for a source file. For any part of a full file
    specification that you do not supply, the debugger uses the file
    specification stored in the module's symbol record (that is, the
    file specification that the source file had at compile time).

    If you specify more than one directory in a single SET SOURCE
    command, you create a source directory search list (you can
    also specify a search list logical name that is defined at your
    process level). In this case, the debugger locates the source
    file by searching the first directory specified, then the second,
    and so on, until it either locates the source file or exhausts
    the list of directories.