You may have missed Jeremiah's post on this yesterday, so let this be a reminder that you should be reading www.sbnation.com/soccer every day for more general newsy stuffs. The fact is that MLS is bringing back the Supplemental Draft a few days after the SuperDraft.
The Supplemental Draft wiki has some background on what it is/was/has been. It is clear that MLS is adding two more rounds of drafting in order to fill-out the 30 man rosters with some Developmental and Reserve level players and not just push some of the Academy kids too hard too early.
So in what is the deepest draft in a while (this was said last year as well), the Seattle Sounders will have 5 picks during the SuperDraft. Those picks will be under the standard roster rules. They will also have 3 picks during the Supplemental Draft. The players picked on that day will be of marginal MLS quality and will mainly see time in Reserve games.
Seattle still needs at least 1 Developmental Player and 4 Reserve Players, and that's just if every returning player makes the roster, and no one starts the season on the injured list. It should be noted that though Julien Baudet's rights are held by Seattle he is currently not under contract. There will be some competition during the pre-season, as the roster is close to "full."