Type forwarding allows you to move a type to other then original assembly without requirement to recompile application that uses original assembly. For instance, if you decide to move one class out of Assembly and you have application referencing this class then in order to ensure that you still can use this application you need to insert Type Forwarding into the assembly you removed your class from.

Put this code inside original Assembly library.dll
using System.Runtime.CompilerServices;
[assembly:TypeForwardedTo(typeof(ClassToBeForwardedLater))]

Than compile original Assembly like that:
csc /t:library /r:newlibrary.dll original.cs