I was recently notified of an opportunity for a TA position. The same email went to the whole department. The position would be helping/teaching a class in a different discipline than mine. I only wanted to ask other people's thoughts about applying because the message specified the only requirements to apply were to be a PGR student in the department. I would ask the faculty in charge of hiring but don't want to look more stupid than I have to. I would think that it wouldn't make any sense for me to apply since my background is in what would appear to be an entirely different field. But maybe I'm wrong?? Shouldn't they have also specified that it's open only to PGR students in that field? My common sense says that they would expect interested applicants to be able to understand that it's implied to be for applicants studying or researching in that field. But maybe there's something about TA positions I'm not aware of?
Some departments need a lot of TAs. Math, for example, since there are a lot of people taking elementary courses. If there aren't enough graduate students in the department to cover the need they might need to look elsewhere, but probably within the same institution.
But this would vary a lot between institutions. I don't find it to be odd, but neither do I think it is common. If you are qualified to assist, or even teach, an elementary course then the position might be open to you. Note that it is probably also true that some departments have unmet financial needs for graduate students and this gives them a way to provide support.