Hi, EKJ. When they ask about what "sources" are you using, they mean things like blu-ray players, TVs, cable boxes, turntables, media streamers, etc. That helps us know what types of connections you'll want.
The Onkyo 818 isn't really a source. It is, essentially, a pre-amp and amp all in one box (along with a radio tuner and other things). You can definitely hook up external amps to it and use it as solely a pre-amp if you want. However, the 818 has a good amp section and should be sufficient to drive your Polks.