Slabs are new. Brand new. You can use the slab on its own, or you can make the slab work with your walls. Making the slabs work with your walls is a huge improvement over the old floor object.
Slabs use styles, just like the wall styles. This will mean that you can set up a library of slab styles that you have created to work with your wall styles, and you will be able to access them from every project.