Frames of reference
The origin is the center of the robot. The robot forward is positive x. The robot left is positive y. The robot turning counter-clockwise is the positive direction for rotation.
Our field reference has the origin on the blue side of the field with the X axis running along the length of the field and the y axis running the short side of the field. This frame doesn't change regardless of what alliance our robot is on.
This means for example that an auto program driving in field co-ordinates would need to use different co-ordinates on red vs blue.