Which parts of a Moentrol transfer valve need to be exposed?
Do not cover over the following items:


Integral Stops (Item 9 on page 2 of the instructions, there are two) These are the isolation valves for the hot and cold connections to the valve. These valves allow you turn off the water to the shower valve to do repair/maintenance without having to turn off the water to the whole house.
Balancing Spool (Item 8 on page 2 of the instructions) This devise prevents temperature fluctuation due to pressure changes to the hot or cold lines, like when a toilet is flushed. It is basically a couple cylinders with small ports that can slide back and forth to maintain the pressure balance between the hot and cold. However, they can become stuck and may need to be cleaned or replaced in time.
Cartridge (Item 3 on page 2 of the instructions) This is what the shower lever will attach to. Again, this may need servicing at some point in the future. Also if you install it incorrectly or if the hot and cold lines are crossed, it can usually be flipped to rectify the problem.
Escutcheon Screw Holes (Item 5 on page 2 of the instructions, there are two) This is where the screws that will hold the escutcheon or trim plate are screwed into.


Page 5 of the instructions you linked to shows that the opening in the wall should be 1/8" larger then the plaster ground face (Item 4 on page two). This opening will be sealed by the escutcheon or trim plate. This will come as part of the trim package. If you provide the model number of the trim package, I provide further information.