Preferred by skiers who want a taller liner. Each liner size can accommodate a range of foot sizes just like a pair of cotton socks. It takes molding heat and pressure to stretch them out for a performance (lower volume/compressed) fit. Because the Universals are taller they have more stretch. For example - I mold out a size 7.0 Universal for size my size 11 US foot.
If you have a lot of space or volume in your boot shell, then order the larger size which will give you more foam. Molding temperature in a pre-heated fan or convection oven is 250DegF (thermometer verify), with a bake time of up to 10 minutes.
Universal Liners may vary slightly on fabric texture, inside/outside colours depending on size and availability.
Liners are sold individually. For a pair, order a right and left liner of the same size and usage.