We're working out what we think - we'll add our view here!
Mountain Hardwear says:
You can SPF all you want, but without UPF in your fabric, you might just end up with the kind of sunburn not even aloe can soothe. This short-sleeve was made for long treks through exposed valleys when your shoulder straps have rubbed off the sunscreen even your sweat couldn't get. With sun-blocking technology built into its fibers, the Wicked Tech™ has you covered.