The supplier is the source of a font, typically a (digital) type foundry.
You should use the supplier letter which matches the supplier you obtained the font from, not the original source; for example, Avant Garde was designed by Herb Lubalin for ITC, but Adobe also sells it. The name of the font that you get from Adobe should start with ‘p’. This is because font resellers typically make modifications to the original design.
Notes on specific suppliers:
For fonts that are distributed without any specific attribution to the creator, by individuals, or by small foundries. (Unfortunately, we don’t have enough characters to assign one to every font supplier in the world.)
obsolete; specifies raw fonts, in the old distribution of Dvips. New fonts should never use ‘r’. (The right thing to do is specify the correct encoding, variant, or whatever the font’s characteristics actually are.)
for fonts that just don’t fit well into the naming scheme. The ‘z’ should be followed by the real supplier letter.
Here is the table from the file supplier.map. It is organized alphabetically by abbreviation. Each line consists of an abbreviation, directory name, and comment.
0 fontfont FontFont 2 elsnerflake Elsner & Flake 5 softmake Softmaker 9 unknown a autologi Autologic b bitstrea Bitstream c cg Compugraphic d dtc Digital Typeface Corporation e apple Apple f public small foundries g gnu Free Software Foundation h bh Bigelow & Holmes i itc International Typeface Corporation j microsft Microsoft k softkey SoftKey l linotype Linotype m monotype Monotype n ibm IBM o corel Corel p adobe Adobe (‘p’ for PostScript) q texgyre TeX Gyre fonts from GUST Typefoundry r - raw (for [obsolete] use with virtual fonts) s sun Storm Type t paragrap ParaGraph u urw URW v sil SIL w wolfram Wolfram y arkandis Arkandis Digital Foundry z - bizarre (fontname is nonstandard) - autofont Eddie Kohler’s autofont program - jknappen Joerg Knappen - mnm Hong Feng, free software in China - yandy Y&Y