What are the differences between 'Outdoor plywood' and 'Marine Plywood'?

Any plywood experts out there ?
I was recently asked by a customer to help them advise a client of theirs on how to seek recourse against a contractor who had installed a hoarding that was prematurely delaminating.
It was a tricky one because the specification was at best unclear and at worst non-existent, there was no contract as such or even letter of agreement and the records were patchy in the extreme.
However, a key point seemed to be that the contractor had said he would use ‘Marine Ply’ which he clearly did not use. He says in hindsight that description was simply an ‘error’ and that ‘Outdoor ply’ had in fact been used.
All sorts of questions arise from this but my first one was “how is the difference between these two defined / classified?”