What if your external data source is not SQL Server or a web service, and is some custom .NET assembly or a flat file? This is where the .NET Assembly Connectors come into picture. Developers can build & design their custom model & entities using Visual Studio 2010, thereby help the BCS to interact with the custom data sources. The .NET Assembly Connector is nothing but a .NET assembly that acts as a proxy to interact with the external data source.
via chakkaradeep.com
Technology Commonplace Blog 