de8fd92a_Advanced_Typography_tables__Reading_support
[0] 2 [name] pair adjustment [Writing support] 'kern' feature s may be created from the Metrics View. 'vkrn' with Metrics->VKern From HKern. [Reading support] 
This sub-table allows the font designer to change the metrics of a specific pair of glyph. The most common use of this is for kerning where the advance width of the first glyph is altered depending on which glyph follows it. But the table is more general than that and could support mark (accent, vowel) positioning over a base glyph (though that is more efficiently done with the mark to base subtable).
Advanced Typography tables
Reading support