You will see on the Internet that people equate sling bags with belt bags (or belt bags, belt bags, belt bags, belt bags, whatever you want to call them), but sling bags and hip bags are not the same thing. Let's repeat it: the back bag and the waist bag are not the same thing.
In short, the belt bag is a renamed belt bag, while the shoulder bag is different (is a fashionable cousin, remember?). A waist bag can almost always be a shoulder bag. Although not necessarily a good handbag, a shoulder bag is actually not a shoulder bag (just like a square is a rectangle, but a rectangle is not a square).
That is to say, almost every company has its own definition of belt bags and belt bags. To make things more complicated, some companies use the term "crossbody" to describe what we think of a shoulder bag. Therefore, it becomes messy—our rules and definitions are by no means absolute.