Besides playing back entire REX Loops using the Run function (or Play in the sequencer), it’s also possible to play individual slices of a loop from a MIDI master keyboard. This way you can use the Dr. Octo Rex almost like a traditional sampler, playing separate slices from separate keys.
The slices are automatically distributed in semitone steps, with the first slice on MIDI note C1, the second slice on C#1 and so on, with one note for each slice. The note range differs depending on how many slices the REX Loop contains.