The EXS24 mkII can loop playback of either an entire sample or a portion of it, when sustained MIDI notes are received.
Control-click either field to open a shortcut menu that allows access to the Sample Editor (or an external editor). This enables you to set the loop start and end points graphically: Loop Start is represented by the LS marker and Loop End by the LE marker. See Editing Samples in the EXS24 mkII Sample Editor.
Note: The ideal settings for the Xfade and E. Pwr parameters depend on the sample material. A loop that cycles reasonably smoothly is the best starting point for a perfectly crossfaded loop, but a crossfaded loop does not always sound better. Experiment with both parameters to learn how, when, and where they work best.