Reference

reference means nothing. Marketing babble.

MK1,MK2....MK...N is a model sequence #-r. Literally it means Mark1...2...
As to Reference I agree with Teejay mostly in one true definition i.e. the most expencive component among all produced.

I and my audiophile friends only use the term "reference" regarding a companie's flagship product that the company believes is the finest peice they can produce at the present time. For example, ML reference 33 mono-blocks, their most expensive/finest piece that they build. The only other way we use the term is regarding our personnal reference as a baseline to measure new pieces of gear against in our systems.