The class of the result is the same as the class of the first argument. If there is just one argument, and it's mutable, a copy is returned.
i1 : join({1,2,3},{a,b,c},{7,8,9}) o1 = {1, 2, 3, a, b, c, 7, 8, 9} o1 : List
The operator List | List can be used as a synonym.