This is not a honorary title, this is an official full diplomatic appointment with a 5 years diplomatic passport from the sending state. This is one of the most popular Honorary Diplomatic Appointees.
The difference between a regular ambassador and the Ambassador at Large is very simple. The usual Ambassador is appointed permanently in a foreign country. For example, the Ambassador of France to Germany. The Ambassador at Large is appointed by the sending state not for a specific country but to a specific mission.
Honorary Diplomatic Appointees
As someone is not a permanent resident Ambassador to a specific country, the Vienna Convention on Diplomatic Relations (of 1961) only has two Articles which cover this subject: They are Article 27 and Article 40.
For 27, someone has to be issued in addition a letter by any mission or consulate or the ministry that they appoint someone as diplomatic courier. This gives special additional protection when someone travels.
For 40, this means that someone is always protected as diplomat when he travels through a third country.
So to summarize, the Ambassador at Large position is not a honorary position, it is a full diplomatic appointment, which, however, don’t need any accreditation of the receiving state, as an Ambassador at Large is not appointed in any specific country.
The full title and Appointment is printed also inside the diplomatic passport, and in addition, a letter of appointment from the Foreign Ministry of the sending state.