If someone has a question regarding certain information on a visa application or certain visa requirements, is this relevant to Travel.SE? For instance, "What documents does a US citizen need for a student visa to Hong Kong?" or "Am I considered of Indian origin if my grandparents used to hold Indian citizenship and have renounced it, but I am of Indian ethnicity?"
I would definitely say it's on topic. Of course, we (the moderators) / the community (more-so these days) check whether there are duplicates.
The other thing I'd recommend for such questions to watch out for is to check that there aren't multiple logical questions being asked in the same set - that would likely get closed as "too broad".
I think it's fine to help, but as far as what documents people should submit for a given application, the guidance is much better. For example if somebody says "I am planning to submit 3 months of bank statements and my tenancy contract". The only thing you can do is compare it to the guidance. What good is that?
The 'official' guidance for the UK and Schengen is wonderful. I worked on the UK guidance during the consultation phase (they sent several rounds of drafts to the legal community last year) and it says exactly what applicants should do. We should avoid competing with it.
And for stuff like sponsor letters or invitation letters, we should avoid having a role in composing them. Also avoid if somebody submits an invitation letter and asks us to approve it, or make suggestions to it, then NO. It is legal advice and outside TSE's remit.
Generally what it amalgamates to is that when an application is in the preparation stage, it's difficult to improve upon what the EU or the UK provide already. And if a question involves something not in the guidance, then there is a very thin surface before it becomes legal advice. Be careful.
Refusals are more straightforward. For refusals, if the OP puts in the formulae (or checkmark), it's ok. If they do not, then to hell with them, it's "UNCLEAR".
At the moment, Schengen refusals are in a state of convergence with very near duplicates starting to appear. If it continues, questions on Schengen refusals can be handled mechanically by pointing to one or more canonical answers. I reckon the entire Schengen surface can be addressed with a max of 5 canonical answers. About 3 canonical answers would be ideal.
For appeals, if somebody wants help preparing their appeal bundle, I vote that this is totally out of TSE's remit. TSE should avoid a high Google rank for appeal questions; mods should delete the question immediately without all the voting rigmarole.
Every other kind of visa question is in scope as long as it is not an Expat thing.