The credits system on ed2k network is between 2 peers only. This means the credits is between you and another peer and nobody else will know about it (or care for that matter).
So, if you're sharing file "fileA" and you uploaded a huge sum of it to user "peer01", you're gaining credits on that user only. You'll benefit from this credit if you're downloading a file which "peer01" is uploading! If you don't need anything from him/her, your credit is lost (after 14 days I think).
(On BitTorrent network, on the other hand, you do benefit from uploading to other peers even if you don't need any part (or pieces) from them as long as you all download the same ".torrent" file.)