Prior to .NET 4.0, when a COM component was referenced, Visual Studio would create an additional assembly to describe the COM DLL to the CLR (a PIA or Primary Interop Assembly). In .NET 4, you can instruct the compiler to embed type information from an i

Using embedded interop types reduces the size of your application, because most applications do not use all the features of a COM component

The compiler embeds only the type information that your application uses and does not embed the unused methods

When .NET client obtains and releases a reference to a COM object, it is required to maintain the reference count on the COM object similar to any other COM client