FString EngineAssociation
The engine to open this project with. Set this value using IDesktopPlatform::SetEngineIdentifierForProject to ensure that the most portable value for this field is used.
This field allows us to open the right version of the engine when you double-click on a .uproject file, and to detect when you open a project with a different version of the editor and need the upgrade/downgrade UI flow. The normal engine version doesn't work for those purposes, because you can have multiple 4.x branches in various states on one machine.
For Launcher users, this field gets set to something stable like "4.7" or "4.8", so you can swap projects and game binaries between users, and it'll automatically work on any platform or machine regardless of where the engine is installed. You can only have one binary release of each major engine version installed at once.
For Perforce or Git users that branch the engine along with their games, this field is left blank. You can sync the repository down on any platform and machine, and it can figure out which engine a project should use by looking up the directory hierarchy until it finds one.
For other cases, where you have a source build of the engine but are working with a foreign project, we use a random identifier for each local engine installation and use the registry to map it back to the engine directory. All bets are off as to which engine you should use to open it on a different machine, and using a random GUID ensures that every new machine triggers the engine selection UI when you open or attempt to generate project files for it.
For users which mount the engine through a Git submodule (where the engine is in a subdirectory of the project), this field can be manually edited to be a relative path.