Oracc, OSL, and other Oracc sign lists aim to be completely conformant to the Unicode standard while supporting paleographic fidelity down to the level of individual handwriting as well as the highest degree of automated cuneiform typography that is reasonably possible.
Oracc uses the Unicode Plane 15 PUA from F0000-FFFFD as a space to create signs pending possible encoding, and to maintain variant forms of signs that do not merit separate encoding ("impossible" numbers like 4(eše@c) would fall into this category, for example). All management of the Oracc PUA is consolidated in the OSL project and is further described on the Oracc PUA page.
An IVS consists of a two parts: a sign and a variant selector. Oracc uses IVS to support encoding structurally distinct graphic variants of the same sign in such a way that both the identity of the sign and the identity of the structural variant are preserved. IVS provides a means of preserving the integrity of paleographic differences without requiring additional character encodings.
For example, the DE₂ sign U+12423 can be written in several different ways: |UMUM×KASKAL| (the reference glyph in the Unicode Standard); UMUM@s; and |UMUM×PA|. If a font supports IVS sequences such as u12423_uE0100=|UMUM×KASKAL|; u12423_uE0101=UMUM@s; u12433_uE0102 then it is possible to encode the data so that the Unicode sign U+12423=DE₂ is still the primary data element, but the way that it is written is also preserved. See the Oracc IVS page for further details.
For conciseness and legibility Oracc documentation notates IVS in font character naming form, e.g., u12423_uE0102. In some contexts an abbreviated notation is used, i.e., [CHARACTER-NAME].eNN, were NN are the last two digits of an IVS selector.
The Oracc implementation of this feature is immature and presently non-compliant with the Unicode standard.
Oracc uses OpenType SSET and SALT for other kinds of graphic variation as described on the OpenType Features page.