Specialists are alternative assignments for Citizens, which involve working a district tile instead of a normal tile. As a general rule, specialists are a good way to get yields otherwise hard to find on normal tiles, but the city needs a solid support base of Food to support them in the long term. Specialists are also particularly useful when a city grows large later in the game, and has more Population than there are normal tiles to work. There are different types of specialists, each with different effects. All specialist types except the generic Citizen have a limit on how many assignments there can be in a city. This limit is determined by the number of buildings in the district associated with that specialist type. Check the building's page to see if they add to the district additional Citizens yields.
Scientists work in the Campus and provide +2 Science.
Priests work in the Holy Site and provide +2 Faith.
Commanders work in the Encampment and provide +1 Production and +2 Gold.
Captains work in the Harbor and provide +1 Food and +2 Gold.
Merchants work in the Commercial Hub and provide +4 Gold.
Engineers work in the Industrial Zone and provide +2 Production.
Artists work in the Theater and provide +2 Culture.