On topic (usually)
I would generally consider a question on-topic if the question is about software that runs natively on Ubuntu, especially if it is in Ubuntu repositories. In these cases, I would assume that they are using Ubuntu unless they state otherwise.
So for example, questions about using Firefox and LibreOffice would be on topic.
If the question is a highly specialized question pertaining to a specific application, it should be judged on a case-by-case basis, even if the software is in Ubuntu repositories. Some questions may pertain to software that runs on Ubuntu, but the question is less about Ubuntu than it is some other specialization.
For example, if someone wants a detailed explanation about how to compose certain types of documents in LaTeX, or if someone has a technical problem with designing a 3D image in blender, we are probably better off directing them to a different SE site or some other forum or Q&A where there is more expertise on those questions.
But similar questions may be considered on topic, like questions about getting certain hardware to work with blender or installing software that can render LaTeX.
Now, if a question is about software that doesn't even run natively on Ubuntu and they don't mention Ubuntu, I would consider the question off-topic. For example, questions asking about Photoshop or Microsoft Office are off-topic.