Well both really. There are 6 bolts in total that drop through each assembly. The outer 4 have nuts underneath, and they join the two U-bars with the main assembly. The inner 2 bolts attach to a lower bracket that wraps around your roof rack. For disassembly from the roof rack, or adjustment, you just need to loosen the knobs - the other 4 bolts stay in place.I'll try to add some more photos to show the situation.