Since TurnKey 12.0 all Turnkey GNU/Linux appliances are based very closely on Debian. Previous versions of TurnKey were based on Ubuntu. Except for a few custom free software components, TurnKey appliances are assembled from unmodified Debian binaries straight from the official Debian repositories.
In other words, under the hood a TurnKey appliance is really just a version of Debian that we optimized for a specific usage scenario by carefully pre-integrating a selection of Debian components. This provides users with a starting point that is closer to the end-result they want (I.e., compared with installing Debian from scratch).
One of the advantages this provides is that all the wealth of Debian documentation applies to TurnKey GNU/Linux. This generally holds true for Ubuntu documentation as well, since Ubuntu is a derivative of Debian.
The main exception is that TurnKey already comes pre-installed so the Debian installation guides don't apply.