In the case of tolerated errors, the service representing the function may be unavailable at numerous times. A single service on a single computer would be an example of an errors tolerated quality of service. If the machine goes down, the service is unavailable.
The service is available most of the time but on occasions errors are known to happen. The exact threshold of error tolerance, the number of errors per unit of time, is service specified. There may have to be manual intervention to correct from an error state.