It is not just semantics!
The PRR had "round roof" boxcars. They differ from the "wagon top" cars the B&O built.
A round roof car is just that a boxcar with a round roof added to provide some extra interior clearance.
The wagon tops were unique to the B&O. The built covered hoppers, boxcars and cabooses with this design. The wagon top design has no top seam in its sheet metal running the length of the car on each side at the roof. The round roof cars have a conventional seam there.
As conventional cars age, the seam between the roof and side of the car has a tendency to leak (rain...), which can result in cargo damage. The wagon top design does away with that seam all together, so it can't leak there. Another design benefit was the fact that the design utilized less steel in its contruction as compared to conventional designs.
I do not believe that the B&O actually had any "round roof" cars. The B&O did re-build some boxcars with a unique truncated coffin-lid-like roof (Mansard roof) to add interior clearance, and these 40 foot boxcars were designated M-27f. (Sunshine Models makes a resin kit of this version)
Westerfield, Sunshine and Funaro & Camerlengo make resin kits for wagon top boxcars. The Funaro kit is marketed under some other distributor's names (North Shore, Central Hobby SUpply)
Funaro also did a kit for the wagon top covered hopper. Pacific Mountain Shops did wagon top caboose resin kits.
It's NOT semantics, and it'sNOT a dumb question.