The Upper Realm consists of extremely large super worlds and other giant planes of existence. They make up the last 5% of the planes of existence in the universe.
The highest existence the Upper Realm allows is unknown.
If the universe was a sky, mountain, and sea, then the Upper Realm would be the sky.
The Immortal World would be an example.
Middle Realm
The Middle Realm consists of large worlds and mini-worlds in the universe. They make up approximately 10% of the planes of existence in the universe.
The highest existence the Middle Realm allows is Great Ascension(or equivalent).
If the universe was a sky, mountain, and sea, then the Middle Realm would be the mountain.
The Spiritual World would be an example.
Lower Realm
The Lower Realm consists of all small worlds in the universe. There is an undetermined number of small worlds but small worlds make up for 85% of the planes of existence in the universe.
The highest existence the Lower Realm allows is Nascent Soul(or equivalent).
If the universe was a sky, mountain, and sea, then the Lower Realm would be the sea.