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:
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.
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 k softkey SoftKey l linotype Linotype m monotype Monotype n ibm IBM o corel Corel p adobe Adobe (`p' for PostScript) s sun Sun t paragrap ParaGraph u urw URW z - bizarre (fontname is nonstandard)
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