The Revit Platform API is installed with Revit Architecture, Revit Structure, and Revit MEP. Any .NET based application will reference the RevitAPI.dll and the RevitAPIUI.dll located in the Revit Program directory. The RevitAPI.dll contains methods used to access Revit's application, documents, elements and parameters at the database level. The RevitAPIUI.dll contains the interfaces related to manipulation and customization of the Revit user interface.