This value is used by the Simulate Particles node. It will simulate particles only for the remainder of the frame. When done, the Simulate Particles node sets this value to 1.0 to avoid having the particle simulated again by another Simulate Particles node later in the same tree or another tree.
The Emitter compounds automatically set SimulatedFrameFraction to (1 – Age/SimulationStep) so that on its first frame of existence, a particle moves only for a distance proportional to its age. As a result, it appears as if particles have been emitted continuously throughout the frame instead of all at once.
You can modify and set this attribute, for example, if you want to change states after a mid-frame event such as a collision or randomized timer, and then continue simulating in the second state for the rest of the frame. This helps to avoid issues with stratification and discretization that might occur if particles changed state all at the same time within a frame. Use values between 0.0 and 1.0 only.