Sometimes, font generators will need to gen this lookup type.
This is needed if the total size of the subtables exceeds the 16-bit limits of the various other offsets in the 'GPOS' table. In this specification, the subtable stored at the 32-bit offset location is termed the “extension” subtable.
Sometimes, font generators will need to gen this lookup type.
https://docs.microsoft.com/en-us/typography/opentype/spec/gpos#lookuptype-9-extension-positioning