I studied at a vocational school some years ago. When it came time to look for jobs, a teacher said that as a foreigner for visa reasons, I would have no choice but to search for jobs in the same industry I've been studying. Which was okay with me since that's what I wanted to work with.
When we were getting closer to graduation, a fellow foreign schoolmate who had yet to find a job basically told me "Don't trust what the teachers are saying to us foreigners. They don't really care about us, only about keeping their 'successful foreign graduates' numbers up". I just figured he was frustrated at his situation and didn't think more of it.
I ended up finding a job in my industry. They didn't know much about work visas since the only foreign employees they had up to that point were permanent residents. So most of what they learned about work visas came from me, and I mentioned the "can't work elsewhere" part at some point in passing. Maybe six months or so after graduation I learned that the previously mentioned schoolmate was working in a place which had nothing to do with what he had been studying, and I never really thought more about this whole thing.
My contract with my company will be ending soon, so this week I went to Hello Work for foreigners to register. After the woman at the desk went through my application, she asked a bit about the vocational school I had gone to and said "You know, you can only look for jobs within this industry, otherwise you can't renew your visa". I asked about some related-but-not-the-exact-same kind of places, but she just said that it wouldn't be possible. I can understand that this could be the case when changing directly from student visa to work visa, but now I'm "just" going from one employment to another (and my residence card doesn't specify anything other than 技術・人文知識・国際業務) but according to her, the immigration office would see my study history.
Now, I don't mind working in this industry, and since this is my Japanese work experience, it's probably by far the easiest for me to find a new job, but am I really "stuck" in that (at least until I can get permanent residence, I guess)? If I were to come across a car mechanic or hair dresser or whatever who were willing to hire me and sponsor my visa, would the application automatically be denied? And if so, then how did my former schoolmate get a job in a completely unrelated industry?