The guidelines say that "questions about small business that have no bearing on personal finance" are off-topic, and also request that "if you have a question about a small business, please make sure it directly involves personal finance." This previous meta question/answer suggested that some questions related to small independent businesses (e.g., one-person businesses) would be on-topic, on the theory that "they relate to a fundamental decision all individuals have to make: 'How will I make a living?'". However, it's been suggested that this question about excise taxes is off-topic, although it seems clear to me from the question that the asker is an performing independent services as an individual for some third-party.

Is this on-topic or not?

My own feeling is that virtually anything related to the internal workings of a business consisting of one person should be considered on-topic (and probably the same for businesses consisting of, say, a husband and wife). I say "consisting of one person" rather than "sole proprietorship" because I really mean businesses where it's just one person doing everything (or at least, questions that would apply in that situation). If your business is just you, everything you do with your business is your personal finance, because who else's finance could it be? Even if there are administrative differences between you and the business, ultimately all decisions are made by you and involve only you.

It's true that there are administrative peculiarities to managing your personal finance when you have a business, but I don't see how that makes this intrinsically "not personal finance" any more than the administrative peculiarities of collecting Social Security, making use of tax-deferred investment accounts, or handling a deceased relative's estate, all of which seem to be uncontroversially on-topic.