Appellation

Overlord

 * Ruler to the monster race.

High Priest

 * Ruler of a single celestial race. There is no ruler to the celestial race as whole unlike the others.

Headmaster

 * Ruler of human Swords Gate.

Realm Emperor

 * Ruler to a Six Paths World.

Sovereigns & Monarchs
There are three titles for one supreme leader in the ancient Chinese culture:

(1)    What are you, the highest-in-command, resembling?

 * Heavenly Monarch (天帝) — The supreme leader who represents the Heaven itself.
 * Heavenly Son (天子) — The Son of Heaven, implemented after the end of Earth Order, when Heaven comes into existence.

(2)    What era are you in?

 * Heavenly Sovereign / Sui Sovereign (天皇 / 燧皇) — Granted to the highest-in-command, Heavenly Monarchs of the Fire Order Era.
 * Earthly Sovereign (地皇) — Granted to the highest-in-command, Heavenly Monarchs of the Earth Order Era.
 * Human Sovereign (人皇) — Granted to the highest-in-command of the human race in the current era where Heavenly Monarch was taken down by Heaven and Heavenly Son is now implemented to the highest-in-command.

(3)    What defines you specifically from the other sovereigns?

 * Great Sui
 * Nong Sovereign (aka. God of Agriculture)
 * Tai Sovereign

Eg. The creator and first leader of Fire Order Era (Heavenly/Sui Sovereign), and also the highest-in-command (Heavenly Monarch), the world have given him his personal, '''unique title of Great Sui. ''' — Heavenly / Sui Sovereign, Heavenly Monarch, Great Sui.

Note: When Sui Sovereign undirected to anyone else, it is commonly referred to the first Sui Sovereign, Great Sui.