Akai iMPC - Manual (Page 7)

16 Levels is a famous MPC feature that allows you to use all 16 Pads to
alter parameters of one Sound. You can use this to play melodic baselines,
create interesting dynamic variations, and more.
To use 16 Levels, play a pad then press the “16 Levels” button. Now all 16
Pads will play the selected Sound at one of 16 distinct values for the
selected Parameter. Experiment with with all four Parameters:
Filter
Velocity
Length
Tuning
Tip: The “natural” value of the selected Param is always on Pad 4.
Variation Slider