What defines a camera as a "professional" model? If the manufacturer calls it a professional model, it is one. .

The only practical result of the manufacturer defining a camera as "professional" is that some manufacturers (through distributors) provide additional resources and services to working professional photographers using "professional" cameras.

As an example, in my area Canon has a loaner program for their high end L series lenses. If you are a working professional photographer (with proof that fulfills their specific requirements) with at least two Canon digital bodies and at least one of those bodies is designated as "professional" you can arrange for short term, low cost or free loans of their more exotic lenses.

I don't know whether a professional user of a Canon EOS-1V would qualify for the program