“root of the project’s source” is added automaticly and you include examples shows that, but does not search recursively (thats what i meant). Compiler (because we talking here about compiler feature not UE4) only search for matching paths in configured include paths. If you look up in engine source code which work same way as game code (game module is just another engine module, it is in seperte forder so you can have multiple project sepretly, but it works the same and practicly you extending engine code from it) it also uses full parths, most common example “GameFramework/Actor.h” which is included in all AActor based classes. It is UE4 convention, but if you like you can alter include path list in build script.