Question about BC21 6sn7 tubes

I was wondering if someone out there could direct me to a reference that would explain to me the differences between the various 6sn7 tubes. I just bought a BC21 and am new to tubes. Is a 6sn7 the same as a 6sn7w, 6sn7gta, 6sn7gtb, vt-231?

I did a search in the forums and found some bc21 owner's favorite 6sn7 tubes from which I can kind of deduce substitutions but thought maybe there was a more definitive list somewhere.

Many thanks in advance.