The different names, numbers only have to do with the country of origin, or whether they are military or consumer tubes. For example: 6SN7, VT231, and 5692 tubes can all be used for the same applications. Some of the lettering on some tubes has to do with maximum voltage and wattage. Again for example the 6SN7:
5692 - 275 Volts maximum & maximum 1.75 Watts per section
6SN7GT/WGT -- 300 Volts maximum & maximum 3.5 Watts per section
6SN7GTA/GTB - 450 Volts maximum & maximum 5.0 Watts per section
If you used a 5692 tube where you really needed a GTA, it would run hotter and not last nearly as long. (Assuming it does not blow out quickly). A GTB used in a lower voltage situation would work perfectly fine and would last a long long time since it would run cooler. The GTA/B I think were designed for the higher voltage needs of televsions, etc.
There is some information on tube basics, matching, biasing, etc, and what types are similar, etc (like above) at www.thetubestore.com
http://www.thetubestore.com/tubereviews.html