As Orcas targets/supports various .NET Framework versions installed and that could have been achieved by installing one .NET Framework version on the top of another version. For example, .NET 3.0 installed on .NET 2.0, and then .NET 3.5 on .NET 3.0.

The .NET 3.5 Framework version holds green bit assemblies which are additional assemblies and can be installed at the top of other existing .NET Framework assemblies.

Green bit assemblies when installed, they don’t affect existing assemblies. For example, .NET 3.0 when installed on .NET 2.0, it does not affect .NET 2.0 assemblies.

In the similar manner, .NET 3.5 assemblies does not affect the framework version either 2.0 or 3.0 on the top of which you will be installing it. You can see in Figure 2-2 that .NET 3.5 implements Green bit assemblies in the Microsoft .NET Framework 3.5 (Pre-Release Version) folder.

Whereas Red bit assemblies are the assemblies which ship either as part of the platform or part of a development tool. For example, Windows Vista ships WPF, WCF etc, and Visual Studio 2005 ships .NET 2.0. Besides assemblies delivered as service packs, hot fixes or updates are also considered as Red bit assemblies.