See: ${parameter@operator} Parameter transformation. The expansion is either a transforma- tion of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter: [...] If parameter is @ or *, the operation is applied to each posi- tional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.
In the last sentence, "the case modification operation" should be "the operation".