We can see it in the help, in IEC 61131-3 Automation platform > Engineering > Target System Configuration > Uploading projects source code.

You can send the source code at the same time as the compiled application (t5.cod) is sent. If the option « download source code” is selected, the workbench will zip a copy of your application folder and download it every time you transfer the binary application to the target. After, you will find a file t5.upl on your device, this file is the zip of your project folder. If the option « download source code” isn’t selected, you will download only the compiled application (t5.cod) executed by the runtime.