Each file in ed2k consists of serveral chunks (parts of 9.28MB).
Image a file consisting of 4 chunks. You have downloaded the chunks 1 and 2. Now you won't need other people who have chunk 1 and/or 2, but not the chunks 3 and 4.
Imagine further you download a small file consisting of one chunk. Another person who has only chunk 2 of the first file has the complete file 2.
a) You won't ask the person for file 1 anymore and you have to wait until server/KAD/source exchange show the person as a source for file 2.
b) You will ask the person for file 1, but not for file 2.
c) You won't ask for file 1, but for file 2.
I recommend c) for obvious reasons.