Anything above Build 7100 is a winmain build. But because they have just split Windows 7 into two as they transition to the RC branch, the current winmain build is older than the current RC build. The RC builds have their code frozen, they will continue to become better, and they will strive to become bug-free builds, they will continue below Build 7100. From what I read, Build 7100 is the assumed official Windows 7 RC1 build. It has yet to be thought out and compiled, but the RC branch ends at Build 7100, so they can stop before Build 7100 and keep the RC1 build at Build 7095 or 7099, but most likely they will go all the way to Build 7100.
From what I understand, Microsoft will continue to make winmain builds, these builds will be much better because their code isn't frozen like the RC builds are. The problem with having a divided Windows 7 is the sync between them. The RC branch might be better for reliability and polished looks, but the winmain branch will have newer code and that means more stable. The point of the RC branch is to take the last winmain build and make it look prettier, no code is really changed, the builds will go higher and higher, but its the same winmain build from before except it is more refined everytime it is compiled as a new build. I'm assuming they sync the latest build from the RC branch with the latest build from the winmain branch, but because they are two different branches, the builds do not come out at the same time and so that is what throws off the winmain builds.
The winmain branch is newer than the RC branch, I will use a winmain build over an RC build any day except this one since it is just labeled a new winmain build, nothing different happened. The winmain builds come after RC and I think having newer code is better than having a little new effect unless a new UI is coming and they decide to throw it in the RC branch and the latest winmain build does not get that yet.
