What are "extraordinary circumstances" for filibuster? Sen Nelson's view

Senate pact shapes high-court fight When "moderate" senators from both parties last month made a deal to end the filibusters agaisnt Circuit courts of appeals appointments of Janice rogers Brown, Priscilla Owen and William Pryor, the democrats promised not to filibuster again except for extraordinary circumstances. What does that mean? According to Senator Nelson (news, bio, voting record) (D) of Nebraska, "I'm leaving open my judgment as to what 'extraordinary circumstances' involves." He says there will be "no surprises," and that members of the Gang of 14 have ongoing discussions on this point.