Larger agencies grow in part by absorbing smaller ones. Cross Country and American Mobile are by far the two largest healthcare staffing agencies in existence and together control close to half of all travelers with their many brands. This include not only their own travelers, but also other agency's travelers. Both agencies have exclusive contracts with individual hospitals. When they cannot meet the needs of these hospitals with their own travelers, they subcontract with other agencies to provide travelers. This exclusive arrangement is also called vendor management and the agencies that hold such a contract are called prime vendors. You can read more about prime vendors here.