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:
f
     r
     r. (The right thing to do is specify the
correct encoding, variant, or whatever the font's characteristics
actually are.)
     z
     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
     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)
     r -             raw (for [obsolete] use with virtual fonts)
     s sun           Storm Type
     t paragrap      ParaGraph
     u urw           URW
     w wolfram       Wolfram
     z -             bizarre (fontname is nonstandard)
     - autofont	Eddie Kohler's autofont program
     - jknappen      Joerg Knappen
     - mnm           Hong Feng, free software in China
     - yandy         Y&Y