The application code is the machine code that a PLC executes when you start an application.

CODESYS automatically generates the application code from the source code that was written in the development system. This is done automatically before downloading the application to the controller. Before the application code is generated, a test is performed for checking the allocations, the data types, and the availability of libraries. Moreover, the memory addresses are allocated when the application code is generated.

You can click “Build  Generate Code” to explicitly execute this command. This is useful for detecting any errors in your source code, even when the PLC is not connected yet. The errors are output in the message view in the "Build" category.