Speaking about that, what is a Celestial Governor? They are members of the Middle Kingdom to which the emperor has granted special privileges, giving them some nice bonuses:

Countries can join the Middle Kingdom freely as long as they are not a subject, and either have their capital in Asia or have the appropriate cultural or religious groups. Subjects of the Celestial Emperor, however, will automatically join it.

Once a country is inside the Middle Kingdom, they will still be allowed to pursue their own diplomacy and wage their own wars (as long as they are not a subject type that forbids that), but they will also benefit from the protection of the Emperor, who may come to their assistance if they are attacked by an external threat (although that is not a guarantee). Also, they will be participating in the Tribute System.

Historically, the tribute system of China was manifested through tribute missions between the various countries and the current ruling dynasty, conducted at various frequencies. Countries would offer gifts to the Emperor for the Son of Heaven to recognize their rule, and they would get gifts of greater value in return.

In the game, this back and forth is simplified and abstracted to a payment that the Emperor has to perform, and the resulting money is divided among all members, according to their economic power. The emperor gets a slider in their economic panel to determine exactly how much tribute they are willing to pay, at the risk of losing Celestial Authority if the resulting tribute is too low.