Some instruments in Cassini TIMs have mechanical RF relays and step attenuators that wear out after being switched millions of times. The available life left is based on the specified lifetime for each component, usually 10 million switches. Each component has it's own life expectancy built into the system.
The wear levels are displayed when the tester is started.
When the wear level reaches 70% of the available life, a full Preventative Maintenance and Calibration procedure should be scheduled. When the wear level reaches 95% of the available life, or if the performance degrades, the TIM containing the Relay or Step Attenuator should be serviced.