When set, PhysicsDistanceJointDefinition.distance is recomputed at create from the world separation of the two anchors.
It is measured after any PhysicsDistanceJointDefinition.autoAnchorA/PhysicsDistanceJointDefinition.autoAnchorB resolution, so it reflects the anchors actually used. It is applied at create only; the authored PhysicsDistanceJointDefinition.distance is ignored while this is set.