The intent of the sticker is to let you know that excessive exposure to direct sunlight will possibly cause the thin wood layer to "yellow" (UV does this) and could cause some drying/cracking of the very thin 0.6 mm (approx. 0.024").
This does not mean you cannot take your board out on a sunny day, and sail it.
If you are at the beach all day in hot sunny conditions, it makes sense to cover your WOOD construction
board with a cloth or towel to keep the sun off it as much as possible when it's just laying on the beach.
If your Carve 131 has the white bottom, then if you must leave it in the sun, I would leave it bottom up, but there is still going to be some degradation (UV) and perhaps some yellowing over time.
If you have a board bag, (even a very light day bag), it makes sense to slide the board in when it's not in
actual use out on the water.
Hope this helps,