Since TurnKey 12.0 all Turnkey Linux appliances are based very closely on Debian. Previous versions of TurnKey were based on Ubuntu. Except for a few custom open source 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 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.