[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.