UPF stands for Ultraviolet Protection Factor.
According to skincancer.org, UPF 50+ fabric actually blocks out UVA and UVB rays.
The sun has UVA, UVB and UVC rays. Typically, we tan by UVA rays, and we burn with UVB rays.
SPF is completely different and only prevents exposure of UVB rays in minutes. With SPF, it is within only the matter of minutes before exposure can take place.
With UPF Protection, it is woven into the materials itself.
Our UPF 50+ fabric is said to block out roughly 99% of UVA and UVB rays!