Share Only the Need
Share Only the Need Hides chunks already gotten by queued clients. eMule works out from what clients it has seen in queue if every one is missing the same chunks, it will hide all chunks that at least one client has got and show chunks that no-one has.
This is only used for complete files.
If no chunks need hiding, SON will turn off (if HideOS is on, it will trigger on, SON will trigger First if both are on).
Hidden chunks are shown in purple in the spread bar with SON (purple is hidden, white is not hidden and black is what the other client has).
Share Only the Need will always make sure chunks that no-one has got get uploaded first so they can be shared with other clients faster. (Also other clients will always see a complete source with SON, with HideOS, it is not seen complete until it has uploaded the file to all clients so many times).
Worst case with HideOS: the clients you've just uploaded 100Mb or more to might have gone offline or stopped the file so the other clients must wait for HideOS to rotate the chunks that have been hidden, whereas SON will just upload the chunks that all clients are missing (it can be used with HideOS as well, SON will start first then HideOS will once everyone has a chunk of your file).