Office Solution Loader

This actually contains two loaders a runtime loader and a soluton loader.

Runtime Loader - (VSTOEE.dll) This runs first to check that the correct version of the VSTO runtime is installed and loaded for the version of Office that is loading the add-in or document level solution.
VSTOEE stands for Visual Studio Tools for Office Execution Engine.
Although multiple versions of the VSTO runtime can be installed on the same computer, only one instance of the VSTOEE.dll is used.

C:\Program Files (x86)\Common Files\Microsoft Shared\VSTO\ VSTOEE.dll 
C:\Program Files\Common Files\Microsoft Shared\VSTO\ VSTOEE.dll 

Solution Loader - (VSTOLoader.dll) This loads the actual solution after checking that the solution assembly is installed and loading correctly.
This creates a new application domain for the solution. It also performs the necessary security checks to verify that the solution has permission as well as loading the corresponding Office Extensions for .NET Framework that is also required.
It implements the IDTExtensibility2 interface which is a COM interface that all Office add-ins must implement.
It also implements the IManagedAddin interface which was introduced in Office 2007.

C:\Program Files (x86)\Common Files\Microsoft Shared\VSTO\10.0\ VSTOLoader.dll 
C:\Program Files\Common Files\Microsoft Shared\VSTO\10.0\ VSTOLoader.dll 

Application Level Loading

Document Level Loading

Other Assemblies