Synchronization is the process of telling a module who its master controller control is. Once synced to a 18R2 Controller, the 18M will only respond to that 18R2 for security purposes. That means, if you are on a shoot site with more than one 18R2, you can feel confident that someone else's 18R2 will not communicate with your 18M. If you sync the module to a different 18R2, the module will forget the original controller and will no longer respond to it. Once synced, there are several security parameters and encryption techniques that exist to ensure the modules will only react to the SYNCed 18R2.
Please visit the 18R2 Controller User Guide: Synchronization to learn how to sync your controller to your module and Audio Box, how to bulk sync modules, and more.
When you sync a module to the 18R2, it is assigned an address by the controller; the address is similar to a serial number that is unique to each module. Addresses begin at A00 and increase accordingly for each module synced to the controller, e.g., A00, A01, A02, etc... To view the 18M address, press and release the TEST button on 18M to display the address on the main display.
The firmware version you use will determine the total number of modules that you can sync to the 18R2. For example, firmware version 5.1 or lower has a limit of 200 addresses, and firmware 6.0 or higher has a limit of 400 addresses. Once you have met this limit, you need to unsync the 18R2 to reset the address counter back to A00 and then re-sync the modules. For more information on the limitations and how to unsync your controller, see How do I unsync my 18M from my 18R2?