When opening a compiled database with the Windows versions of 4D, 4D Server, 4D Runtime, or 4D Engine, a file is created which has the same name as your structure, with the name ending in the ".CMP" extension. This file is created in the same folder as the structure. It is required to run the compiled database. This file is created the first time the database is opened. If you delete this file, a new one will be automatically created the next time you open the database. This file is actually a DLL which stores all the runtime libraries used by compiled code generated using 4D Compiler.