Self-selection typically yields a great fit - people have a good idea if their foot is pancake flat, small arch or cavus (high arched). If still a little unsure we recommend:
Low: for those with a completely flat foot or with a very minor arch but looking for the insert to primarily provide some additional comfort while riding.
Moderate: minor to moderate arch height. Can still be tolerated by those with a flat foot but recommendation for those is no higher than this.
High: moderate to high arched foot. Not recommended for those with an especially flat foot.