The plane normal is supposed to define the true "up" direction for the reference, since the reference itself can rotate freely.
Also since the plane is calculated based on male colliders, during animations it could rotate, so settings a static plane in a scene can make the calculations be more stable.
Currently its only used for L1/L2, but I will also make it so its used in R0/R1/R2. Right now R01/R1/R2 are calculated based on reference-target orientation.
Calculating R0/R1/R2 based on target-plane orientation should improve device movement further, this was suggested by
@TBD