Hopper cars, as such, are not a category.
To quote from the 'horse's mouth';
E - Open, high-sided wagon, of ordinary type;
F - Open, high-sided wagon, of special type (i.e. suitable for UK, &c.);
G - Covered wagon, of ordinary type;
H - Covered wagon, of special type;
I - Temperature-controlled Wagon;
K - 2-axle Flat wagon, of ordinary type;
L - 2-axle Wagon, of special type;
O - Mixed Flat and Open High-sided wagon, or ordinary type;
R - Flat Bogie Wagon, or ordinary type;
S - Flat Bogie Wagon, of special type;
T - Wagon with opening roof;
U - Special Wagons, other than Categories F, H, L, S or Z;
Z - Tank Wagon, with metal shell.
All of which are subject to qualification by the 'index' letters (e.g. Zi would indicate a Tank Wagon with a non-metallic shell).
https://www.era.europa.eu/sites/default/files/registers/docs/appendix_6_p12_en.pdf explains it all!
Pat