Also called Telephone Service Factor (TSF). This is the percentage of calls answered within a specified amount of time. For example, if the service level time is set at 50 seconds and 85% of calls are answered in less than 50 seconds then the service level is 85. The actual amount of time specified is chosen based on a variety of management considerations.
This is sometime abbreviated 'SVL' when display space is at a premium (such as on call center readerboard systems).