ProSite free trail level


I have a quick question regards the free trial level option on ProSites, we don't have a free level all our levels are paid. It was my understanding that the user will just get a free trial of the level they select at the checkout. So what is the need for this option, does it mean if they pick the top level but this option is set to the lowest they will get given a trial of the lowest level even though they picked the top?