Where does "handbook" fit in?

[matevz]

There is another type of documentation common particularly in the FOSS world for desktop applications called "handbook". For example the KDE project has a collection of handbooks for their apps at https://docs.kde.org/

Outside of CS I handbooks are also common in the aviation.

Where do you think this type of documentation structure fits in the diataxis framework? Perhaps somewhere around how tos and references?