I don't get this pigeonholing of the concern as being only about "water hobbyists" or "underwater professionals". It's literally just a type of machine you will become aware of some time during the course of reading about engineering or subsea operations. As I said, I'm not even fully sure it just applies to underwater vehicles yet I have bumped into the term/concept more than several times already in my life. If you're new and this is your first time encountering it, appreciate the learning opportunity. I bet you'll see the term a lot more now due to Baader-Meinhof.