What is lumen maintenance?

Well understood that LEDs are very long life light sources. However, the light output of SSL products gradually goes down over time. Rated lumen maintenance life of the LED (L p) is the elapsed operating time over which the LED light source will maintain the percentage, p, of its initial light output 

L70 (hours) = time to 70% lumen maintenance 

L50 (hours) = time to 50% lumen maintenance