An OpenType font can have up to twenty Stylistic Sets, indicated as ss01, ss02 .. ss20. Oracc uses SSET for collections of variations which correspond to usages typical of a period, styles related to a medium, or general handwriting styles. For example, Oracc uses ss01 for early writing styles which exhibit certain mergers that take place immediately after the Fara period; ss02 for middle period mergers; ss03 for later mergers and splits. Sset ss04 is reserved for monumental script--so a Neo-Babylonian font could have sign forms typical of writing on stone in ss04, for example. Sset ss05 is reserved for cursive style writing.
An OpenType font may have up to ninety-nine character variants. The following explananation of CVNN is to be found in the Learn/Microsoft Typography description of Tag: 'cv01' – 'cv99' [https://learn.microsoft.com/en-us/typography/opentype/spec/features_ae#tag-cv01--cv99]:
The function of these features is similar to the function of the Stylistic Alternates feature ('salt') and the Stylistic Set features (see 'ss01' – 'ss20'). Whereas the Stylistic Set features assume recurring stylistic variations that apply to a broad set of Unicode characters, however, these features are intended for scenarios in which particular characters have variations not applicable to a broad set of characters. The Stylistic Alternates feature provides access to glyph variants but does not allow an application to control these on a character-by-character basis; the Character Variant features provide the greater granularity of control.
The kinds of character variation that Oracc might use with CV--stacking pattern variations, character composition variations--can also be supported with IVS (see above). Because of the 99 CV limit imposed by OpenType fonts it is preferable to support these kinds of variation through the IVS mechanism.
It is likely that Oracc will use CVNN to provide alternate groups of sign-forms in certain fonts. For example, an Early Dynastic font may have reference forms in the primary set, Fara forms in ss01, and Abu Salabikh in ss02. In addition, important groups of texts such as the Blau Monuments or CBS 10000 and related documents could have distinctive sign forms implemented via the CVNN feature.
Oracc uses SALT to support creating fonts which contain examples of minor variations in form that may be specific to an instance, a scribe, or a school--many sign lists that are restricted to a particular period or corpus demonstrate several variant forms of signs, and a font that emulates such a list can use SALT to encode these variants.
SALT variations are font-specific and do not need to be aligned or harmonized across fonts.
There are several kinds of OpenType ligatures, but Oracc consistently uses the LIGA feature ("Standard Ligatures") which are automatically selected in a font when the components occur in sequence.
Some ligatures are defined to work only when there is a U+200D, Zero-Width Joiner (ZWJ) character between the components. For example, this allows the sequence diŋir-en to be treated as two characters or a single ligatured character as required (scribes sometimes mix their use on a single tablet).