Table Directories
A Table Directory is an index to the tables in a font. It is unique to each font. In a TTC file, there are multiple Table Directories: one for each font to be enumerated from the TTC file.
A TTC file Table Directory has exactly the same format as a TTF file Table Directory. The offsets in the Table Directory are measured from the beginning of the TTC file.
Each TrueType table in a TTC file is referenced through the Table Directories of all fonts which use that table. Multiple Table Directories may reference the same TrueType tables; these are "shared" tables.