When set, both local anchor frame rotations are recomputed at create so the slide axis is the direction from anchor A to anchor B, preserving the bodies' relative rotation.
It is measured after any PhysicsSliderJointDefinition.autoAnchorA/PhysicsSliderJointDefinition.autoAnchorB resolution, so it reflects the anchors actually used. It is applied at create only; the authored anchor rotations are 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.