Can someone explain what this file is and what it's purpose is? I've seen it referenced a couple of times but no one seems to give a good explanation to what it does.
The purpose of variant/* files is to describe other boards that use the same Core.
For example. A "custom_Due" might wire the digital pins to different Port pins on the chip. Or have a different amount of Flash. Or different USART or SPI wiring.
If you look at the SAMD21 core, it has many variants.
Google "Arduino board.h" or "Arduino variant"
David.