"Reflex" (in this case) refers to the upward angle of the airfoil/aileron at the wing tip.

All flying wings need reflex to offset the fact they have no elevator . Elevators provide a moment to stabilize the lift of the wing. Reflex does this for flying wings. (Tailess aircraft)

It is the consensus of the Roo pilots that 1/8" of up elevon at neutral is what the this aircraft needs to fly like the trainer it's supposed to be.

Hope this helps, not just to confuse you further,