TT represents the typeface name. W represents the weight.
Many fonts have more than one variant.
DD represents the design size, and is omitted if the font is linearly scaled from a single TFM file. The weight, variant, and width are probably all best taken from the original source of the typeface, instead of trying to relate them to some external standard. Before giving the lists of abbreviations, let me point out some problems. I don't know of a good solution for any of them. Please let me know if you have ideas about any of them.