Automatically-Generated Build ID
At build time, every output directory that contains at least one compiled DLL receives a JSON file with the ".modules" extension, such as "UE4Editor.modules". This file lists each Module in the directory and the associated DLL for that Module, as well as a GUID for the build itself. Every time the Engine compiles, it generates a new GUID, so that DLLs from other compilation sessions will be recognizable, and the Engine can ignore them. When using source control to maintaining a binary build, you must check in the executable, all DLLs, and their associated ".modules" files together to ensure that the Build ID matches.