Many of these pan configurations are different and may vary, but when I did mine, the difference was filled in with mortar, and the level of the mortar had and must configure with the bottom of the pan to give good and proper stability when you step on it.
I am on a slab also. Then I also lined the area with that heavy, I mean heavy drain pan membrane that is another safety feather to funnel water to the drain if the pan fails in any spot.
This is a pretty intricate project and if not done correctly, you could have nightmares with future water leaks and unsupported areas in your pan not filled in with mortar.
Make sure the mortar does dry properly and effeciently. Very important.