Where do they come from and what do they do?

Or is it all mostly a myth.
Please a real answer. Seems like emule tries to avoid the subject

You do not have credits for yourself on your machine, everyone else stores your credits. Every emule client tracks every other emule client and the amount of downloading and uploading, and there's a slight queue ratio multiplier that's applied based upon that ratio. I don't think every client has an implementation of it.

The exactly mulipliers are probably documented @ http://www.emule-project.net

That's one of the reasons why its beneficial to upload as much as possible.