I didn't find documentation on this so I'll provide a little howto.
When you are adding a single node it's easy enough to identify the polling engine you want to manage this node.
But if you want to run a network discovery job, you can run it off a specific polling engine. If you have multiple polling engines in your environment a dropdown to choose the polling engine will be at the top of the first page of network discovery options.
Either way you can select the polling engine you want and the discovered nodes will be added to that polling engine.