I'm a bit confused. I used to think that it's good to provide additional information from other sites whenever an answer is submitted but later I realized it's wrong, but not totally. Now another question is up. If I have decided to give references or citations, I'm (almost) always confused again about what should I give. So far I only provide links to Wikipedia and a few online dictionaries (like Cambridge's) but I often see various sites linked to. Is there a "somewhat" clear instruction on choosing off-site resources, reference targets and citation sources?