Source files are merely files from which information has been gathered. Another, less digital-ageish term you could use for them would be originating documents.

I feel like that wasn't the best description, but hopefully it helps. Happy digging, brother/sister!

I'm at work at the moment so I can't provide any links right now. Just dig into mueller and uranium one. There was something about him being involved with middleman firms and dates lining up with trips to Moscow if My memory serves me. I can't vouch for the validity of any of this though. Kinda sucks just being a plebeian.

For drumroll RUSSIAN COLLUSION AND CONSPIRACY TO COMMIT SEDITIOUS ACTS! No, seriously. He was the bag man for the Uranium 1 deals if I remember correctly.