I cannot say anything about coaches in the UK, but was asked by someone on this thread about the tourist_bus vehicle class on the access page. The wording is a literal translation of a sign I found here: https://www.openstreetmap.org/way/100307268#map=16/42.2460/11.7783
The term used on the sign (free form extension sign) was tourist bus which isn’t a category in Italy according to the highway code. Given that there are only 2 types of busses in the wiki, bus for busses operating as public transport vehicle, and tourist_bus, I would expect the latter to describe coaches.
I would not look at weight ratings but rather on the maximum capacity of the vehicle: if it is intended to transport more than 8 people it is a bus.