Confused about ABus. Please help!

In your case it sounds like A-Bus is a good solution to extend audio into the Dining room and Master Bath.

A-Bus subzones will provide independant on/off and volume control for these two rooms and will allow you to listen to any of the six sources. You'll only be able to select the first four sources from the A-Bus keypads, but you can select source 5 or 6 from the UNO keypad located in the same zone to play them on teh A-Bus subzone.

To add the two A-Bus subzones you'll need two A-KP2 keypads and one A-PS power that will plug into the back of the CAV. You can only connect A-bus subzones to the first four zones on the CAV6.6, so during your planning be sure the Kitchen & Master Bedroom zones are destined for one of the first 4 zones. Wiring is CAT5 cable from the CAV6.6 to the A-Bus keypad location and then 16awg or 14awg speaker wire from the A-Bus keypad to the speakers (16awg is all that required because the runs are short and it's much easier to work with).

There's some more information in the following thread... take a look at post #10 for some additional A-Bus info:
http://www.avsforum.com/avs-vb/showthread.php?t=769435

Speaker brackets installed during construction will make installing the speakers a snap! Not really necessary, but is much easier than trying to find all the wires and getting the speakers positioned properly after the walls are up. Another great advantage of the speaker brackets is they will allow you to properly line up in-ceiling speakers with recessed lighting during construction. If you don't get this sorted out before the ceilings go up, you'll probably find that some of the recessed lighting will be installed directly against a ceiling joint which may not allow you to properly center a speaker with them due to their larger diameter.

Cheers,
Paul