Future extensions to GX may use the descriptors for services such as cross-font style matching. Font developers should assign values to fdsc tags with user functionality in mind. Any additions or changes to that list should be reflected here. A nalf which is zero or undefined means the font is alphabetic. A style-matching scheme might look first for a matching nalf font type, and only then attempt to match the other font descriptors.
Optical size is the output size which is best for the design of the font.
For a family with multiple design sizes, the opsz descriptor could be used for style matching. In the absence of other information, the recommended optical sizes for Roman fonts are 12 point for text and 24 point for display. For upright Roman fonts, slnt should be 0. Width is a ratio relative to the center of the family. The regular or default face has, by definition, a width of 1.0.