An item that is unique to a particular unit or formation, thus allowing its presence to be inferred from the presence of the signature equipment. Against foes that organise themselves properly, such as the dear departed Warsaw Pact, they allow ready identification of Divisional Artillery, Army level recce assets and the like. This technique is however of questionable use against a bunch of men all seemingly named Achmed with big moustaches clutching AK-47s and RPGs.
Signature equipments are why Intelligence Corps personnel engaged in combat Intelligence activities get really excited about obscure box-bodied vehicles with bizarre antennae. Or at least that's the explanation they will admit to.