There are three Selection related buttons:
Play Selection, to only play the selected segment. In combination with Loop Mode the selection is played multiple times. Use the Selection icon from the selection controls, or use the shortcut key SHIFT+SPACE.
Clear Selection, deselects the current selection. Use the Deselection icon from the selection controls, or the shortcut key ALT+SHIFT+C or ALT+C. You can also use the shortcut key CTRL+SHIFT+Z. This shortcut also cancels selection mode.
“Move crosshair to the left/right boundary of the selection". Whether Selection Mode is on or off, you can move the crosshair to the beginning or end of the selection (Ctrl+/).