What’s the difference between sushi and onigiri?
Again it’s a matter of how things are normally categorized in Japanese cooking, but generally speaking anything made with vinegar flavored rice, or sushi rice (sushi meshi or shari) iis sushi. So you could make an onigiri-shaped item with shari, and call it an onigiri, but if a Japanese person ate it s/he would probably think it’s sushi in an onigiri shape.