Assemblies are the building blocks of .NET Framework applications. An assembly is a collection of types and resources that are built to work together and form a logical unit of functionality. An assembly provides the common language runtime with the information it needs to be aware of type implementations. To the runtime, a type does not exist outside the context of an assembly.
If want to call an external application that exposes itself using .NET Assemblies, you need access to the .NET documentation provided with that application (or contact the software vendor for further information).