Apple  has defined 17 TrueType tables that for now are used only under Mac OS X. These tables are known by the name "AAT" (= Apple Advanced Typography). We shall divide them into three groups.
The first group, to which this section is devoted, contains eight various and sundry tables. The second group, with five tables, is dedicated to fonts with variation, a concept close to Multiple Master. Finally, the third group, with four tables, contains the big guns of the AAT tables—i.e., tables for positioning, substitution, and justification; they use finite automata to do their task, which makes them very powerful—and very difficult to design.
But let us begin with the relatively simple tables: acnt, bsln, fdsc, feat (with the list of predefined AAT features), lcar, opbd, prop, trak, and Zapf.
This table's name comes from the word "accent". In fact, it is a table for describing composite glyphs, in the same vein as AFM files (§C.3.7) or the virtual fonts of TEX (§B.3). Composite glyphs consist of a base glyph (here called the primary glyph) and one or more combining glyphs (here called secondary glyphs). The table contains the names of the glyphs in question and their relative positions.
Rather than giving a global offset for the combining glyph (as in AFM files or TEX virtual fonts), the acnt table adopts a more efficient strategy that is also closer to the type 4 lookups of the GPOS table: a point on the base glyph's contour and ...