Hueco Mundo is another world that exists inbetween the Soul society and Earth where the Hollows exist. the world is divided into several different sections beased on the strength of those that preside within it.

Muchos
This is the largest section of Hueco Mundo where Hollows in general exist and live closest to the barrier to the human world. they however typically dont travel to the other sections of Hueco Mundo since the ones here are typically weak and will be killed by the Arrancar or the Menos that exist in deeper sections of Hueco Mundo

Grande
30% of Hueco Mundo is Grande. it is the deeper section of the dimension and it is where the Menos Grande reside. Menos Typically stay dormant inside of this area, not usually leaving unless they wish to travel to another of the worlds which usually will only occur if a strong spiritual pressure reveals itself on earth

Noches
The final 20% of Hueco Mundo is known as Noches. in the deepest, darkest section of Hueco Mundo is This area, resided in by the arrancar. they are the strongest members and have their side closest to soul society so that they may travel there when they wish. also inside of Noches is a large temple where Sosuke Aizen used to control the Hollow from. Arrancar usually stay here but are free to travel to other sections of soul society as they do not fear any of the other residents of Hueco Mondo