A destination management company, or DMC, is a local agency that arranges venues, transport, staffing and activities for an event in a city it knows well.
A DMC is hired for local knowledge. They source venues and restaurants, arrange ground transport, book offsite activities, supply staff and handle permits, all in a city the organizer may never have worked in. On an international program they are also the people who know which supplier will actually turn up.
A DMC handles the destination. A professional congress organizer handles the conference itself, including the program and registration. The venue handles the space and usually the catering. On a large event all three are in play, and the boundaries need setting in writing, because the overlaps are where things get dropped.
Either a management fee, a commission on the suppliers they book, or a mix of both. The commission model is common and worth asking about directly, because it shapes which suppliers get recommended. Asking for a fee-based arrangement is normal practice on larger budgets.
The threshold is unfamiliarity rather than size. A small executive retreat in a city you do not know benefits more from a DMC than a large conference in a venue you have used for five years. Groups that need transport, offsite dinners and activities coordinated to a schedule benefit most.