Source : Free On-Line Dictionary of Computing

TL1
     
        Transaction Language 1.
     
        A subset of {ITU-T}'s {MML} from {Bellcore}, with simpler
        syntax.  TL1 is similar to {USL}.  It is used in
        communications between telephone operating systems and remote
        network test equipment.
     
        [OTGR, TR-TSY-000439, section 12, Bellcore].
     
        (1994-12-14)