I think customizing Ubuntu is a way of using Ubuntu. In particular, it is a way of making changes to an Ubuntu system.
If someone wants to add Debian repositories to an Ubuntu system, we help them with that. If someone wants to add Ubuntu repositories to a Debian system, that's off-topic here. When someone customizes an Ubuntu system, whether or not they intend to produce a derivative that may eventually be a separate distro, what they have to start with is an Ubuntu system, and they know what changes they have made.
(This is, interestingly, perhaps the only situation where people are likely to know all the changes they've made to their Ubuntu system.)
In contrast, if someone starts with some unofficial Ubuntu derivative, we probably don't know how it's different from Ubuntu, they probably don't know, and there are people in other places that do know.
As far as I can see, we have a lot of good reasons for considering questions about using unofficial derivatives off-topic, and none of them apply to customizing Ubuntu (whether or not the intention is to create and distribute a new operating system):
Experts on the derivative should be consulted instead.
There are no experts, because the derivative doesn't exist yet. The OP knows what they want, and we know how Ubuntu works. That's the best there is. (And it's pretty good.)
Users should seek help in the right community.
There is no community, because the derivative doesn't exist yet.
We would overstretch ourselves to provide support for other OSes.
There is no substantial technical distinction between a customization that is a separate distro and one that is not. Once a distro takes off, then it might be possible for us to overstretch ourselves. But then that distro exists, has a community, and so forth.
We're not familiar with the best pre-existing documentation.
There is no pre-existing documentation for the custom distro because the custom distro doesn't exist yet.
It's not Ubuntu!
That's the goal, not the present situation. Once someone succeeds at building an unofficial derivative, then it makes sense for that derivative to be off-topic.
We don't have to help; someone else will.
When you build a good OS, users will come. Before you build a good OS, you rely on your knowledge, skills, and whatever help good people are willing to give.
Once modified even slightly, we cannot predict how it will work.
Again, there is no technical distinction between an unofficial derivative and any other kind of customization. We help people make custom live CD's, even though that means that once one thing is changed, the system is different. We also help people modify their installed Ubuntu systems, sometimes in radical ways.
More importantly, we shouldn't provide user support for a custom distro, no matter how young it is.
I think that fundamentally captures the important distinction:
- When there is an unofficial derivative, and you use it, there are many good places to get help with that, but Ask Ubuntu is not one of them.
- When there is Ubuntu, and you modify it with the goal of creating a custom distro, you're using Ubuntu, and Ask Ubuntu is perfectly suited to helping you with that.
- If you modify it in some weird way to create a problem that doesn't exist in Ubuntu, that might merit closure--even if we don't consider it a bug, it might be too localized.
Does that mean if someone modifies Ubuntu during the course of normal use, clones it, and redistributes it, that's off-topic?
Yes, that's a kind of custom distro, and is off-topic.
We can't support an unlimited variety of systems.
Right. So we should give reusable advice about customizing Ubuntu.
When a question has the potential to help people creating many different custom distros from Ubuntu, the question is about Ubuntu.
Using that question as an example, it's asking (and I paraphrase):
- How to make the Software Center use different software sources?
- How to replace Ubuntu One with another cloud service that integrates similarly?
- What Canonical services is Ubuntu built to use, and how do I replace them with my own services?
There are some serious problems with this question. It's not very detailed. It's very broad. It's vague. (Even the third question, as I've broken it down, might not be detailed enough to constitute a good question by itself.)
If the OP doesn't provide more information, we may have to close it as not a real question, for those reasons. So I'm not sure it's the best question to help us figure out a policy about scope (as it may not work out, for reasons other than scope).
But even there, you can see that this question tells us very little about the specific nature of the custom distribution the OP wishes to create. Answers to those questions would be immediately applicable to many or most people interested in customizing Ubuntu.