Hi Pateplov,
From Cisco Unified CCX Administration Guide, Release 8.5(1)
Service Level - The target maximum number of seconds a call is queued before it is connected to an agent. This is a mandatory field.
Service Level Percentage - The target goal for percentage of contacts that meet the service level. This is a mandatory field.
For example, a call center that has a service level of 20 and a service level percentage of 80 percent has a goal of answering 80 percent of its calls within 20 seconds
That is the Service Level Percentage is the target goal for the percentage of contacts that meet the defined Service Level. Even though this is a mandatory field, it only serves as a goal or target level you hope to achieve.
Thanks and Regards,
Geevarghese