I can see this z field for every layers in my tilemap, can it be used to show some layers over and some layers under my character sprite or the only way is making different tile maps for backgrounds and foregrounds?
You have to use two tilemaps. There is an issue on this. DEF-1184
As @sicher says, you must currently have two tilemaps. You can still paint in one and leave the other blank (you need to set the corners though so it gets the correct size).
Then, after loading them in game, you can copy the tiles from a layer in one tilemap into a layer in the other tilemap. Should help with the content authoring at least.
I did that and it works perfectly, thank you