Question

Difference between his highness and his excellency

Answer 1

You use the word 'your' when you are talking to the person directly.
For instance, "Your highness". "Your excellency", "Your grace", "Your honour" etc.
Eg. The carriage has arrived, your highness.

You use 'his/ her' when you are talking about the person in question to a third person. Terms used in such a case would be "His highness", "His excellency" etc.
Eg. His excellency, the Honourable President, is set to arrive here on Thursday.