The on-topic help page states:

We welcome questions about:

Using and administering Ubuntu, including official Ubuntu derivatives.

The link goes to Canonical's list of official derivatives. However, that page ends with a link to the full list of derivatives, most, if not all, of which are off topic.

I found this slightly confusing at first and since on-topicness is often an issue here, perhaps it would be a good idea to emphasize the word official. Something like:

Using and administering Ubuntu, including official Ubuntu derivatives (but excluding unofficial derivatives)

Or even simply

Another possibility would be to link to the Recognized Flavors page on the Ubuntu wiki instead and modify the help page accordingly:

Using and administering any of the Recognized Flavors of Ubuntu

I think that making that help page more explicit would help clarify the scope of the site and might help reduce the number of off-topic questions.