I think it's the effect level that you need. So if you have a summon with effect 3, you need a control of effect 3 or more to control it. 3 would be able to give it suggestions and it might do them. 4 would mean (almost) full control of it. Which means without a ritual, you can't properly control an effect 5 summon (which is fair enough, that's a fairly powerful beast).
