My understanding is that Texas A&M must first announce it's intentions to leave the Big 12, before the SEC will extend an offer to join the SEC. The SEC wants to avoid a lawsuit for basically tampering with another conference's member under that is under contract with them.
Under the terms of the Big 12's contract with ESPN and FOX, the league requires 10 members to survive and fulfill the terms of the contract. That is why the Big 12 has been trying to keep A&M and find a replacement soon-as well as try to keep Missouri in the fold.
Once A&M divorces the Big 12, it can receive an offer to join another conference and take it's action to accept it. Of course, that offer can come from the Pac 10, the Big 10, or from the SEC. I can not see the SEC backing off and allowing one of these other conferences to jump in and take over the footprint that the SEC would really like to have in Texas.
I would surmise that the agreement has been made in aback room between surrogates from the two parties on how things will shake out from here.
I also tend to believe that the SEC has a 14th member lined up and that it would be Missouri. Missouri wanted to join the Big 10 a year ago, but it was not ready to tale on another member at the time. The Tigers want out and have been among the most vocal dissenters of the LHN and the bogarting of the Big 12 pot of gold that has caused this breakup.