A concession granted to the winning project bidder gives the SPV long-term rights to use public assets (land, operating licenses, etc), in return for the SPV being contractually responsible for the full delivery of services. Services can include the operation and maintenance of the assets, and also for financing and managing all of the required investment.
Concessions for usage or operation of assets are also typically coupled with offtake mechanisms. In this case, the concessionaire obtains most of its revenues directly from its users through tariff levels established by the authority in the offtake contract. These can include payment schedules, changes in payment schedules over time as well as events to trigger a review of the payment schedule.
Relying on the "invisible hand of the free market", it is then the prerogative of the concessionaire to achieve improved levels of efficiency and effectiveness since any gains in efficiency translate into increased profits and returns to the concessionaire; although regulators may set additional key requirements such as maintenance and renewal or replacement of assets.
Additional concessions may be given (or adjusted) where the deal economics may be potentially challenging, for example if the aggregate amount of tariffs collected by the concessionaire is not sufficient to cover the cost of operation of the assets (or even maintenance or further investment). In capital-intensive projects where there is a high initial capital outlay, for instance, there has to be some degree of revenue recovery or minimum guarantees (such as availability payments or exemption of operating license fees) to ensure the project company can sufficiently meet interest and debt repayments.