The predicate open_resource/3 first checks resource/3. When successful it will open the returned resource source file. Otherwise it will look in the program's resource database. When creating a saved state, the system normally saves the resource contents into the resource archive, but does not save the resource clauses.
This way, the development environment uses the files (and modifications) to the resource/3 declarations and/or files containing resource info, thus immediately affecting the running environment, while the runtime system quickly accesses the system resources.