NEW: Manafort's agreement with DOJ requires him to "cooperate fully, truthfully, completely, and forthrightly," including participating in debriefings, providing all relevant documents, and testifying if the government asks him to. 
Manafort signed the plea agreement Thursday.
The sentencing guideline calculation doesn't work out well for Manafort. Unlike the other Trump aides who've been charged, he's looking at significant prison time. (The guideline range here actually exceeds the statutory maximum by a lot.)
Where that leaves things: Two top Trump aides - his former campaign chairman and his former national security adviser - have pleaded guilty and agreed to cooperate with Mueller. We don't know the subject of their cooperation. But it suggests prosecutors have an additional target.
We don't know the subject of Manafort's cooperation. But recall that last year, Mueller's office obtained a search warrant for his residence that listed among the items to be seized records involving "attendees of the June 9, 2016 meeting at Trump tower."
