Library /sys$common/syshlp/basichelp.hlb Labels  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  A label is a  1-  to  31-character  name  that  immediately  precedes  and
  identifies  a  statement  or  block  of  statements.   The label name must
  conform to the rules for naming variables.  The  initial  label  must  end
  with a colon (:).  However, references to the label, for example, in EXIT,
  ITERATE, and GOTO statements, must not include the colon.