These codes were selected to fit Adobe font catalog, and should be a reasonable guideline for future font design. Ties are resolved in favor of the lighter weight. The attribute is a boolean indicating whether kerning is enabled. In most applications, kerning improves the appearance of text and should be enabled. These ligatures are unusual enough that they should only be chosen when explicitly desired.
Ligatures are also very important for non-Latin scripts.
The attribute value is the additional letterspace to add before the affected glyph, specified in 0.001 em units. The attribute is a boolean indicating whether small caps are selected. When small caps are selected, lowercase characters become small capitals. It might look like THIS . Small capitals are a very nice typographic refinement, and it is recommended that they be used where appropriate.