RON is short for "Run On Network".
DSP:
We strongly recommend to keep selected "RON" check box in the Campaign/Keyword settings as a starter. It will open your campaign to all our sources of traffic.
Then you will be able to target a specific source or to apply another kind of targeting knowing which one perform better for your campaign.
myDSP:
You may select RON for mis intent, instead of targetting domain or keyword, that will bring you more traffic, although, intargeted.