I consider ones with a fixed rear window to be a coupe. So the Del Sol, Corvette C4, late Porsche 911 and cars like the X1/9 and Porsche 914.

Targas with a drop rear window or detachable rear window I would consider to be a convertible, so early Porsche 911 and Lancia Zagato/Beta Spider, as well as the Triumph Stag and versions of the Jaguar XJS that have the targa top.

T-Tops are always coupes.

That's just my take on how they should be classified.