OpenType feature tag

Choosing cvXY and ssXY

Forums: 

Is there any informal standard on how to choose which character variant (cv01 through cv99) or stylistic set (cv01 through cv20) a certain glyph or group of glyphs should go into?
Alternatively, has someone conducted a survey what actual typefaces put in these features?

With [[http://www.w3.org/TR/css-fonts-3/#font-feature-values|CSS Fonts 3]], there is a higher-level language to add mnemonics for these enumerated features (and some more) when used with the font-variant-alternates property.

@font-feature-values FontFamilyName {
@styleset {insular: 1; uncial: 2; unicase: 3;} /* ssXY */
@character-variant {initial: 1; medial: 2; final: 3;} /* cvXY */
@stylistic {cute: 1;} /* salt index */