de8fd92a_Advanced_Typography_tables__Reading_support
[0] 4 [name] mark to base [Writing support] These may be created with the Points->Add Anchor command [Reading support] 
This sub-table allows the font designer to specify how mark glyphs (accents, vowel signs, etc.) are positioned over base glyphs. Every glyph can have an attachment point and the mark's attachment point will be placed on the base's attachment point so the two join properly. See my example in the overview.
Advanced Typography tables
Reading support