Velocity is usually used to control various parameters according to how hard or soft you play notes on your keyboard. A REX file does not contain velocity values on its own. And when you create sequencer track data by applying the "To Track" function, all velocities are set to a default value of "64". As velocity information is meant to reflect variation, having them all set to the same value is not meaningful if you wish to velocity control Dr.Rex parameters.
There are basically two ways you can apply "meaningful" velocity values to REX files:
After creating track data, you can edit velocity values in the Velocity Lane in the sequencer.
You can play slices in real time on your keyboard. The resulting data will have velocity values reflecting how the notes were struck when you played.
When velocity values have been adjusted, you can control how much the various parameters will be affected by velocity. The velocity sensitivity amount can be set to either positive or negative values, with the center position representing no velocity control.
The following parameters can be velocity controlled: