imu inertial measurement unit
The IMU's job is to assist with orientation, pitch and roll. It helps immensely with accuracy in a single-GPS configuration, but it's not needed if you're going dual-GPS. You can use it in dual as well tho.
The BNO085 is the one to go for - no other is as good. The Adafruit is popular, but if you look at any kind of alternative, the pins have to be in the same configuration - so play it safe and stick with this. These can be hard to find, so watch the Discourse or Telegram channels as helpful members often post when they see stock at $VENDOR.
IMU needs to be placed like on the above picture with X or Y matching up with the direction the tractor travels. (you can orient it 0,90,180,270 degrees and configure X / Y + invert accordingly from the software)