how does DSU sideloader work?

I have a question about the userdata size item. I set it to 5GB, as I saw in one of the videos. However, I discovered that 16GB in total is available. So what does this affect? Is it possible to use this system fully? That is, does it have any restrictions? And what if I want to reflash it again, or delete this firmware? Is it possible to place several at once? And how many?