Any question that's specifically relevant to open source works and projects should be on-topic. If “the question only tends to arise in an open-source context”, it's on-topic.
In particular, avoid the temptation of “there's already a Stack Exchange site for that”. The existence of another site is largely irrelevant to decide the scope of this site. Quoting Shog9's blog post “http://blog.stackoverflow.com/2012/03/respect-the-community-your-own-and-others/”:
As members of a community, your first loyalty should be to that community. When evaluating a question, you shouldn’t be looking to push it off on some other site; instead, ask if it could be appropriate and on-topic for you, the experts who the author decided to ask. Be a bit jealous of your site – don’t blithely turn askers away simply because their question could be asked somewhere else. Don’t hit them over the head with your scope, help them tailor their question to fit into it – and if that means your site’s scope overlaps a bit with another site’s, so be it.
Obviously, there are questions you’ll have to turn away, either because their only connection to your site is via the audience (“How do I make bread as a programmer?”), because it’s completely off-topic (“How do I cook a fish in a dishwasher?” obviously belongs on Cooking, not Home Improvement) or because they’re simply not useful or constructive. But that should be your last resort. Close questions with an eye toward improvement and re-opening, not driving users away.
What bears discussion is where to draw the line on questions that are relevant to open source, but not exclusively so. Is this site the one-stop for all topics open source (including generic questions about version control, forum etiquette, licensing, etc.), or is it only for topics that are specifically about open source?
The line boundary may well be different for different topics. There can be good reasons to reject some topics — in particular, if the site turns out to be bad at getting good answers to certain topics, then it can make sense to declare these off-topic, because no answer is better than bad answers. For example, questions about computer security and questions about laws often attract uninformed answers with bad advice; if we get too many bad answers about law or security, we might refuse questions on these topics to avoid being a repository of bad information.
Here are a few examples of inclusiveness and exclusiveness across Stack Exchange:
- Programmers rejects questions about programming careers. For a while, it accepted career-related questions, but only if they applied specifically to all programmers. Eventually, the Programmers community decided to remove career and workplace issues from their scope altogether, because very few were genuinely specific to programmers.
- Unix & Linux accepts questions about cross-platform applications. If you're running a program on a Unix system, it doesn't matter that this program also runs on Windows.
- Unix & Linux refuses programming questions, and refers askers to Stack Overflow instead. This was done because SO was already established when U&L started, and U&L defines its audience as users and administrators, not programmers. Programming is off-topic on U&L because it doesn't match the site's audience.