So let me get this straight (about contacts)

[Einbaum]

I've been playing this game a while now and I still don't entirely undertsand some things. Admittedly, I'm the polar oppostie of the guy who plays a game to death. I linger on a game until it gets obselete and move onto something else having delved into the game 30-40 per cent at most. I hardly ever "finish" games and I doubt I'll ever see a Type XXI. So please bear with me.

1) On the navigation map...those ship icons (which in stock SH3 were either green or red and in GWX are black) which mean a "contact". How do they come about? Is it supposed to be BDU who tells you? And how does BDU know? From the Wermacht spotting ships? Or spies at the port? Or is it something entirely different like the Radioman intercepting radio signals? I've never understood this...

2) How do you exactly send a contact report? Because what I do at the moment is...chase a contact, fire torpedos...if I have no more torpedos I then call in the contact out of spite. But where do they get the contact info from? From my Watch Officer with his visual sightings? From my targetting data? And should I update this info just before sending the Contact Report?

3) Why does GWX replace "BDU" with "FDU" and what is FDU?