Tier destination is a term used in meetings industry which refers to the ranking of city as 1st, 2nd or 3rd destinations within a certain country.
1st tier destination are mostly capital and the biggest convention cities, 2nd are the smaller cities which are more affordable than the 1st tier destination.
Usually these destinations have a smaller number of international flights.
Regarding the 3rd destinations they are usually based on the size of the city or a destination and its convention offerings. For example they can include tourist resorts, which can be a great choice for an incentive trip.