de8fd92a_Advanced_Typography_tables__Reading_support
[0] 6 [name] mark to mark [Writing support] These may be created with the Points->Add Anchor command [Reading support] 
This sub-table is very similar to the previous two except that the base glyph is itself a mark. This may be used when a glyph has two accents each of which would normally be placed at the same attachment point on a base glyph. The second accent will be place relative to the first accent rather than to the base glyph.
Advanced Typography tables
Reading support