Not to be confused with T-Max films, it's a speed-enhancing liquid developer.

Like HC-110, it comes as a liquid developer, although not quite as concentrated. It seems to be a PQ (Phenidone-Hydroquinone) based developer, so it's not quite as speed-enhancing as XTOL, but it does seem to have more true speed enhancement than old MQ (Metol-Hydroquinone) developers like D-76 have.

It's fairly well formulated for pushing. Apparently (and I don't have references for this, so I might be totally off base) the earliest T-Max films had rather poor toe-speed so T-Max developer was made to compensate for this. Modern T-Max films are much better behaved so they work just fine in normal developers, but T-Max developer is still useful for pushing.