What is the coin style battery on the back of the Processor used for?
This battery does not have any bearing on the configuration of the processor. The processor has a built in Astronomical time clock function that is disabled by default. If enabled, the battery simple holds correct time while the processor may be powered down.
This battery is a model CR 2032 coin style battery. It is installed with the negative pole (-) against the metal retaining clip. This battery can be source locally in any number of retail locations.