By positioned correctly I mean directly mounting the pump to the rad, or using a short, wide-bore connection. Not routing 5 feet of 3/8" ID tube round the case to connect it as people are wont to.
Metzen - net positive suction head is basically the amount of pressure on the suction side of the pump. If this is too high it can affect the performance of the pump (particularly with the aquarium pumps we use) and cause caviatation. Adding a res reduces pressure as it's a lot easier to pull water from a big open tank than down a thin pipe.