All of our UV-resistant T-shirts are UPF 50+, i.e. they have the maximum level of sun protection.
The choice between them will therefore be made based on the length of the sleeves and will depend on what it will be used for. The constraints are greatest for surfing, so we decided to build our range primarily for surfing.
For surfing, the choice will be made according to the duration of your sessions: 1 hour, 2 hours or 3 hours.

1 hour: Occasional practice. You can choose between a loose fit (priority to ease of dressing and comfort when dry) or a fitted cut (priority to comfort when wet). A loop system to connect it to the shorts ensures the stability of the product.
2 hours: Regular practice. Comfort when wet is enhanced by the seamless panels under the arms and the drawstring, which further increases stability.
3 hours: Intensive practice. Additional warmth to help you stay in the water for longer.

The minimum water temperature recommended for the use of this UV-protection T-shirt is 72°F if it is worn on its own. Max. duration: 3 hours.
This top is made of fleece, which helps avoid shivering when you enter the water. Please note that the presence of wind will have an impact and can cause discomfort when wet, making you feel colder. In windy conditions it is therefore recommended to choose a top with neoprene.
This top can be worn under a wetsuit

The fabric of this OLAIAN UV-resistant product blocks at least 98% of UVB radiation and 95% of UVA (UPF 50+). This guarantees effective protection from the sun. It can be used both in and out of the water, for water or nautical sports. UV radiation is filtered by the density of the textile fibers, for long-lasting protection, even after washing. Caution: deteriorates in chlorine.

UPF is a sun protection factor (Ultra Violet Protection Factor) and is standardized worldwide.
It defines the protection level of a fabric and more precisely the proportion of UV rays it lets through.
To meet the standards, it must block at least 95% of UV rays. UPF 50+ means that the fabric blocks at least 95% of UVB and 98% of UVA rays.

This thermal top helps limit rubbing and chafing due to contact with the surfboard or bodyboard. It can also be used under a wetsuit to limit chafing, particularly around the neck and armpits. We have added seamless panels under the arms to limit irritation due to paddling movements. Our test protocols confirmed that irritation was indeed limited during 3-hour sessions.

The Tribord UV-resistant top has a drawstring at the hips. This lets you tighten the top so that it stays firmly in place. It has also been designed so that the drawstring of your boardshorts can be attached to it to stop the top from riding up while you surf.

Our UV tops are designed to meet the needs of the most demanding surfers. Our design choices also make them compatible with other water sports such as swimming, kitesurfing, windsurfing, wakeboarding, water skiing, sea wading or SUP (stand-up paddleboarding).

Although this T-shirt protects against the sun, phenomena such as reflections (from water, sand ...), heat, time of day, etc. must be taken into account.
Wearing this T-shirt does not exempt the wearer from adopting suitable behavior: wear sunglasses, wear a hat or cap, regularly apply an appropriate sun block, drink regularly, etc.

https://www.olaian.fr/conseils/la-protection-solaire-en-surf-tp_3890

OLAIAN, DECATHLON'S SURFING BRAND
Based in Hendaye, in South West France, OLAIAN is Decathlon's surfing brand. In our offices at the water's edge, we can design a pair of boardshorts and then on the same day go surfing and test them to check that they stay in place in the waves, are comfortable and durable.