Can someone please explain the difference between these two Tug/Barge combinations.

ITB= Intergrated Tug Barge. The two are rigidly connected. Vessel pitch is the same, only one set of running lights required. Basically considered a ship when joined.
ATB= Articulated Tug Barge. The two are joined with a hinge type coupling. Vessel pitch is independant. Two sets of running lights required. Considered a tug pushing ahead.

ITB/ATB: A convenient way for a company to man a ship as if it were a tug.

This post from the MTVA Forum may offer a bit more detail on this subject…