yInternalLeading (SHORT)
The amount of space which, when subtracted from yMaxAscender, gives a measure of the distance above the baseline that characters extend. This measure is font-design dependent, but glyph-set independent. Therefore, this calculation approximates the visual top to a row of characters without actually looking at the characters in the row.
It is recommended that applications use this field to position the first line of a block of text by subtracting yInternalLeading from yMaxAscender. Then position the baseline that distance below the preceding information (text or graphic element).
Note: This use does not guarantee that characters will not overwrite those above them; it does give a font designer's view of where to place the text. Collision should be tested for and additional space allocated if necessary.