/sys$common/syshlp/helplib.hlb GOSUB, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 label

    Specifies a label of 1 to 255 alphanumeric characters that
    appears as the first item on a command line. A label may not
    contain embedded blanks. When the GOSUB command is executed,
    control passes to the command following the specified label.

    The label can precede or follow the GOSUB statement in the
    current command procedure. When you use a label in a command
    procedure, it must be terminated with a colon (:).  If you use
    duplicate labels, control is always given to the label most
    recently read by DCL.