When getting root access to your device or installing any custom ROMs you need to know the number of your device's firmware. This number does not necessarily have to be the same for the same model of phone. For example, all Samsung Galaxy S3's represent the same model, but do not have the same firmware number.
Firmware is the operating software available on an Android device, and it is available in different versions designed by different manufacturers. Basically it's the device-specific part of the software. For example, you may have Android 4.2.2 running on your phone, but have a firmware number that looks completely different because it relates to more details than just Operating System. The firmware number consists of several elements, all of which are essential for the functioning of the phone:
- PDA: Android operating system and your customizations.
- Phone: the actual identifier of your device.
- CSC (Country Exit Code): the languages and country-specific parameters.
- Bootloader: the boot loader program that runs at startup to all unit processes.
The settings menu on a Asus Memopad device shows the firmware number as Build Number