If you mean XMI that's XML indeed. But merely it using XML doesn't begin to describe its insanity. It has metamodels, meta-metamodels, ..., all described in OMG

If you mean XMI that's XML indeed. But merely it using XML doesn't begin to describe its insanity. It has metamodels, meta-metamodels, ..., all described in OMG's "MOF", starting from the idea that OO principles somehow pass as formal method or logical framework, and ends in "Extreme UML", posing decision problems with unknown complexity/feasibility/decidability left and right. It's truly junior OOP zealotry going beyond its comfort zone. What's particular bad IMHO is that OMG/IBM managed to undermine completely unrelated and useful BPM modelling standards into the dead end that was UML, taking those down along with the nonsensical XMI, MOF, EMF, GEF.