In the context of EU transport law, a PSO is an arrangement in which a governing body or other authority offers subsidies in an auction, whereas the winning company will be obliged to operate a specified service of public transport for a specified period of time for the given subsidy. This usually leads to a monopoly of the winning bidder, as competing services would not be viable without subsidies. That is done if there is not enough revenue for routes to be profitable in a free market, but there is a socially desirable advantage in the transport being available.