Volume Table of Contents question for Dos 3.3

[<F0O@psuvm.psu.edu>]

HI!

   I'm using Dos 3.3 on an apple IIe and need to write out at the sector
level using the RWTS subroutine.  I want to update the track bit maps
in the volume table of contents as I go along.  All so far is fairly
simple.  My question is with 30h - 33h in the VTOC.
   In the Dos manual the description is: "These 4 bytes are a mask for
the track bit maps: each 1 bit enables one of the 16 sectors to be used in
every track."
   I'm not sure why you would need to have a mask for the track bit maps.  It
seems redundant to me.  If in the track bit map, you are marking the sector as
either being free or used, what more do you need?
   Could someone please enlighten me as to what I don't understand.  Thank
you for your support!

                                                            [Tim]