How the maximum service interval is related to UAVs?
Service interval  - is a maintenance term indicating the interval (either in time or number of flight cycles) between two successive services (e.g. inspection, change of oil, etc.) that a component requires. 

In case of UAVs this is not different. It can be for example that the Service Interval of the engine is 100 hours of flight time, meaning that the UAV can fly missions for 100 Hours between engine maintenaince services.