@Sao311tigger
You don't need to assign the back yard to an apartment. Nor do you want to imo, as it will raise the rent because it'll be seen as rented space like the inside of the apartment. As long as the entrance to the back yard is through an apartment, then that apartment's tenants will the only ones that can access and use the space. You can also put a gate in the fence if you want and lock to for just the apartment's household. So, you can have a back yard that is not on a foundation and is lower than the building foundation.
Hope this helps.