Difference between Assemblies Framework and Extensions

Question

What is difference between Assemblies Framework and Assemblies Extensions in Add Reference Dialog box?

Answer 1

According to MSDN (hit the ? in your Add Reference dialog):

The Assemblies tab consists of two sub-tabs:

1.Framework lists all assemblies that constitute the targeted Framework.

2.Extensions lists all assemblies that external vendors of components and controls have developed to extend the targeted Framework. Depending on the purpose of the user application, it might need these assemblies.

So the first is just what Microsoft delivers for that .NET version, the other one contains all others.