uTorrent has a special setting called When Reaches the Seeding Goal then Limit the upload rate to.
What it means is that if you have 2 files:
X - already downloaded 50%
Y - already downloaded 100% (i.e. finished downloading)
Then maybe you'd like to give different upload speeds to each of them - 1 speed for files that are being downloaded (e.g. X) and a lesser speed for files that already finished (e.g. Y).