Just because something's labeled as one thing, doesn't mean it can't be used for something else. A sock can be used as a puppet. A credit card can pick locks. And flooring tiles can, without question, be used on your walls.
Actually, any type of flooring designated for 'the floor' can be used in any vertical surfaces or countertops. In fact, flooring is designed to endure a lot more wear and tear than any wall or countertop would ever endure, meaning that a flooring material might be your best bet for a durable wall design.
We as consumers call tiles 'floor' tiles or 'wall' tiles, but in reality, these tiles are just judged upon certain criteria, such as the PEI rating. For example, a tile with a PEI rating of 1 is only suitable for walls, since it can't handle foot traffic. Thus, this tile would not be considered a 'floor' tile. Tiles are also rated on their Static Coefficient of Friction quality, which is just a way of saying how slippery a tile is. A tile with a PEI rating of 3 and a COF of 5 would be considered a 'floor' tile, but you can certainly put that tile on the wall. It'll just be one heck of a durable wall tile.
Of course the inverse is just not possible. You cannot and should not put designated wall tiles on your floors. That's because a wall tile is likely unable to handle any foot traffic, and it most likely far too slippery for a preferred flooring surface.
Ceramic, tile and slate tiles are quite common installs for walls. One of the major differences you may see between traditional wall tiles and floor tiles used for the wall is the level of glossiness. If you're looking for a super-shiny appearance on your walls, you may want to go with a traditional wall tile. These tiles have a lower friction quality, thus they appear glossier.