Specifies a font subset.
PDF documents may include subsets of Type 1 and TrueType fonts. The font and font descriptor that describe a font subset are slightly different from those of ordinary fonts. These differences allow a conforming reader to recognize font subsets and to merge documents containing different subsets of the same font.
For a font subset, the PostScript name of the font (the value of the font’s PdfFont.BaseFont entry and the font descriptor’s PdfFont.FontName entry) shall begin with a tag followed by a plus sign (+). The tag shall consist of exactly six uppercase letters; the choice of letters is arbitrary, but different subsets in the same PDF file shall have different tags.