The sort function uses <= to compare elements of the list, which in turn calls upon ?.
i1 : sort {c,e,a,f,b,f} o1 = {f, f, e, c, b, a} o1 : List
i2 : sort {4,2,6,3,8,2} o2 = {2, 2, 3, 4, 6, 8} o2 : List
See also rsort, <=, and ?.