When is MonoBleedingEdge used?

I’ve noticed that my Unity4 installation (and my Unity3 installation before it) has a “MonoBleedingEdge” and a “Mono” directory inside the Unity.app (on Mac).

Specifically:

$ ./Unity.app/Contents/Frameworks/Mono/bin/mono --version
Mono JIT compiler version 2.6.5 (tarball Mon Sep 24 16:08:20 CEST 2012)
$ ./Unity.app/Contents/Frameworks/MonoBleedingEdge/bin/mono --version
Mono JIT compiler version 2.11.2 (tarball Wed Jun 13 21:15:42 CEST 2012)

Now, one of them is definitely more up to date than the other - 2.6.5 is pretty old (latest stable is 2.10.9, not sure where 2.11 fits in) - but when is MonoBleedingEdge used, and when is Mono used? I’m pretty sure 2.6 e.g. doesn’t have C# 4.0 and .Net 4.0 support. Is it possible to switch between them?

You might want to ask Unity Support. They generally know more about Unity’s backend