Unfortunately the numbers are basically made up arbitrarily by more than one vendor so you just have to memorize the list

. As someone else said, a TA housing has a bolt on inlet bell and is stock appearing, while a TE housing has the inlet cast as part of the compressor cover, and makes 10-30 more hp but is not stock appearing and needs a modified wastegate actuator. The original TE/TA turbos used the Garrett housing but most now use the Precision 3-bolt turbine housing which gives more top end power than the Garrett housing but spools a tick slower. Sometimes you see letters like PTExx which means the turbo was built by Precision and has the Precision turbine housing, LTxx which means it was built by Limit Engineering, and Txx which indicates Turbonetics (who also uses stage x terminology which I'm going to ignore here). There is a chart at gnttype.org which gives the wheel sizes of the popular models, and definitely give the vendors a call to see what they recommend for your combination of goals, converter, heads, cam, intercooler, and fuel system. Anyway, in order of increasing size (based on wheels and assuming a Precision housing) they go stock, TE/TA32, TE/TA34, TA49, TE44 (same wheels as a 49 and don't ask why the number is different, I told you it is arbitrary

), PTE/PTA51, TE/TA60, PTE52, PTE53, TE/TA61 and PTE54 (same wheels), TE/TA62, TE63 and TE45, TE63E and TE45A, and after that they generally go by compressor wheel size like T72 or LT72 or PT72. And of course these days there are the new Garrett GT wheels entering the picture. The GT350 turbine wheel in different trims is used in the PTE51 and PTE52 turbos so that's no big deal nomenclature-wise since the turbine wheel was the only difference between a 60, 61, and 62 anyway, but now there are a few compressor wheels out such as the GT61, GT67, and GT71 wheels. The GT61 makes things confusing because of the existing TE/TA61 turbos. Real Soon Now (tm) complete Garrett GT turbos will be available with their own naming scheme and this will get even more complicated (as I understand it they are going to use a Precision 3-bolt turbine housing modified to mate with the new center section, and getting this casting done is taking time). So that's a short version of the list - happy researching.