Recommended characters and optional glyphs
One of the aims of SMuFL is to make it as simple as possible for developers both of fonts and of scoring software to implement support for a wide range of musical symbols. Although modern font technologies such as OpenType enable a great deal of sophistication in automatic substitution features6, applications that wish to use SMuFL-compliant fonts are not obliged to support advanced OpenType features.
The basic requirements for the use of SMuFL-compliant fonts are the ability to access glyphs by their Unicode code point, to measure glyphs, and to scale them (e.g. by drawing the font at different point sizes). If applications are able to access OpenType features such as stylistic sets and ligatures, then additional functionality may be enabled.
However, all glyphs that can be accessed via OpenType features are also accessible via an explicit code point. For example, a stylistic alternate for the sharp accidental designed to have a clearer appearance when reproduced at a small size can be accessed as a stylistic alternate for accidentalSharp, but also by way of its explicit code point, which will be in the range U+F400–U+F8FF.
Because optional glyphs for ligatures, stylistic alternates, etc. are not required, and different font developers may choose to provide different sets (e.g. several different appearances of tab clefs, or different sets of glyphs whose designs are optimized for drawing at different optical sizes), SMuFL does not make any specific recommendations for how these glyphs should be assigned explicit code points, except that they must be within the range U+F400–U+F8FF, which is reserved for this purpose and for any other private use required by font or application developers.
In summary, recommended characters are encoded from U+E000, with a nominal upper limit of U+F3FF (a total of 5120 possible characters), while optional glyphs (ligatures, stylistic alternates, etc.) are encoded from U+F400, with a nominal upper limit of U+F8FF (a total of 1280 possible glyphs).
In order for a font to be considered SMuFL-compliant, it should implement as many of the recommended characters as are appropriate for the intended use of the font, at the specified code points. Fonts need not implement every recommended character, and need not implement any optional glyphs, in order to be considered SMuFL-compliant.