@tag: a feature tag @value: 0 disables the feature, non-zero (usually 1) enables the feature. For features implemented as lookup type 3 (like 'salt') the @value is a one based index into the alternates. @start: the cluster to start applying this feature setting (inclusive). @end: the cluster to end applying this feature setting (exclusive).
The #hb_feature_t is the structure that holds information about requested feature application. The feature will be applied with the given value to all glyphs which are in clusters between @start (inclusive) and @end (exclusive). Setting start to @HB_FEATURE_GLOBAL_START and end to @HB_FEATURE_GLOBAL_END specifies that the feature always applies to the entire buffer.