Your professor is right. Don't over think this.
Think of PSV like "car". A car can be an/made by Audi, BMW, Chevrolet, Dodge, Ford, Tesla, Volvo, etc. It can be a race car, jet car, test car, taxi car, etc. It can be a sub-compact, compact, or full size. Car is very general.
A PSV can be/made by Anderson-Greenwood, Dresser (Consolidated), Crosby, Farris, Leser, Sempell, etc. It can be a "conventional safety valve", "balanced bellows safety valve", "pilot operated safety valve", "remote pilot operated safety valve", etc. It can be a safety valve with gas/vapor trim or liquid trim (I don't think anyone has developed a two phase trim yet). PSV is very general; it covers a lot of different safety valves.