This menu item is disabled (grayed) until you specify the extended metrics. The information you can then specify in this dialog box tells the driver how to synthesize effects such as underlining and superscripting, while allowing for the per-font, per-size customization of these effects. It also forms the remainder of the EXTTEXTMETRIC structure in the PFM file.
Superscripting, subscripting, underlining, double underlining, and strikethroughs require two numbers. The vertical position of the effect and its size. Offsets are measured downward from the baseline of the font, which means that strikeouts and superscripts will generally have negative offsets. The size specifies the thickness of lines in font units for the line effects, or the height in font units for superscripts and subscripts. For double underlines, you may specify the upper and lower lines independently.