From the PDFLib site's documentation:
fontname (Name string) The name under which the font will be registered, and can later be used with PDF_load_font( ).
a, b, c, d, e, f (Will be ignored in the second pass of the font definition for Type 3 font subsets) The elements of the font matrix. This matrix defines the coordinate system in which the glyphs will be drawn. The six values make up a matrix in the same way as in PostScript and PDF. In order to avoid degenerate transformations, a*d must not be equal to b*c. A typical font matrix for a 1000 x 1000 coordinate system is [0.001, 0, 0, 0.001, 0, 0].
optlist (Ignored in the second pass for subset fonts) An option list. The following options can be used: colorized, familyname, stretch, weight, widthsonly