YAPP is a certain API level of YAKE.
While YAKE provies general functionality, systems and API abstractions, reflection, serialization, (virtual) file system, etc, the groundwork, YAPP is built on top of these components.
It provides extensible and often generic implementations for common concepts like finite state machines, task management, messaging (not yake::rx events or signals) and the “model” concept which is extended by implementations for vehicles and more.
Take a look at the Components Manual for details on yapp::model or yapp::loader and the other components.