de8fd92a_Advanced_Typography_tables__Reading_support
[0] 1 [name] single adjustment [Writing support] These can be created with the Element->Char Info->Position command. [Reading support] 
This sub-table allows the font designer to change the metrics of a specific glyph. The feature tag will provide a context in which the change should occur. For instance the 'tnum' (tabular numbers) feature could change a proportionally spaced digit by adjusting its advance width to be some set value and then centering the digit (by adjusting the left side bearing) within the new width.
Advanced Typography tables
Reading support