Release Time slider: Define the time required for the sustain level to fall to zero, after the key is released.
Velocity Sensitivity slider: Determine the velocity sensitivity of the entire envelope. If set to maximum, the envelope outputs its maximum level only when the keys are struck at maximum velocity. Softer velocities result in a corresponding change to the envelope levels, with a 50% velocity resulting in half-levels for each envelope-level parameter.
Download the guides:
Logic Pro User Guide: PDF