When set, local anchor frame A's rotation is recomputed at create so the slide axis is the direction from anchor A to anchor B.
It is measured after any PhysicsWheelJointDefinition.autoAnchorA/PhysicsWheelJointDefinition.autoAnchorB resolution, so it reflects the anchors actually used. It is applied at create only; the authored PhysicsWheelJointDefinition.localAnchorA rotation is ignored while this is set. Anchors that coincide in world space (an auto anchor places them together) have no direction, so the world X axis is used.