MTTF is a basic measure of reliability for non-repairable systems. It is the mean time expected until the first failure of a piece of equipment.
For example: Fans’ characteristics

It is to sample one lot of fans for life test and when the failure accumulates to 10% of time, the L10 temperature’s value is equal to MTTF temperature 1/9.5 time.
MTBF (mean time between failures) is the expected time between two successive failures of a system. Therefore, MTBF is a key reliability metric for systems that can be repaired or restored such as airplanes and cars. MTTF (mean time to failure) is the expected time to failure of a system. Non-repairable systems can fail only once such as fans and tubes. Therefore, for a non-repairable system, MTTF is equivalent to the mean of its failure time distribution.
When a life accelerated test is done within 70 degrees, 30000hours (CL 90%), MTTF/MTBF value is a product’ life expectancy, not guarantee all products’ life 100% to be achieved.