The Generated/BuildDef.hpp file is a temporary file where you can define custom definitions. It looks like this:
// Generated file, DO NOT TOUCH!
#undef PRODUCTION
#undef DEVELOPMENT
The first definition is PRODUCTION. If defined, the project will be compiled for production. This should only be enabled when using the production export scripts.
The second definition is DEVELOPMENT. If defined, it enables a bunch of code that can be useful for internal framework debugging purposes by framework developers.