A read-only per-particle attribute called finalLifespanPP stores the final result of lifespan computations no matter what mode you are using. The particle shape uses finalLifespanPP to kill particles and to drive ramps.
You cannot set the value of finalLifespanPP yourself in any way (for example, with an expression, ramp, or component editor). Like age, it is an attribute the particle shape computes for you. But you can read its value (but not assign its value) in an expression.
In Random range mode, lifespans are determined for each particle when it is born, and are stored in finalLifespanPP. If you change Lifespan or Lifespan Random, it affects new particles but does not affect particles that are already born. If you rewind and re-play, all particles are affected.