The term UPF stands for Ultraviolet Protection Factor. It is used as a standard factor to measure the effectiveness of a fabric or canvas against harmful sun rays. In the context of sun shades, UPF signifies how much protection the sun shades provide against harmful rays of the sun. The UPF fabric of 50/50 rating or 49/50 rating is the best option. It blocks 98 percent of ultraviolet rays from reaching you.