Help Desk have agreements with their customers or users these are known as SLA (Service level agreements) this tells the user of what levels of service to expect from the Help Desk.  Although not every Help Desk has a SLA they may be have an informal standard level of service it offers to its customers.  The SLA is formal document written and signed by both the client being served and the provider of the Help Desk.  It gives everyone involved expectations of what the Help Desk will do, defining such things as the hours of service and agreed length of time for resolution of problem.  The service level agreement is used to give an overview to management of what the service will provide.  

This gives the Help Desk a business purpose and helps everyone involved discuss the services available within the Help Desk and helps remove ineffencies.  Bruton (1995) writes that SLA is best used in large organisation were the Help Desk is well organised and were the support is largely repetitive.  For any organisation were the Help Desk has been outsourced a SLA is a must, so the service provider is compelled to provide the level of service they agreed to.