The XL-size is suitable for extremely light wind. Only a small gust of wind is needed for planing. The XL is also perfect for heavier or seasoned sailors who can use the sail in stronger winds to maximize power in combination with smaller board shapes.
The size L is great for lighter winds. It adds another option depending on the current conditions. For example, in gusty or choppy conditions, this sail size combined with a small board shape can give just the right amount of extra power so you don’t get overpowered.