Every Attribute (both calculated and initialized) must specify its type.
For most expressions, the type is either fixed (e.g. the all expression always returns a Boolean) or can be inferred (e.g. a reference returns the type declared by the referenced Attribute).
However, for some expressions the type must be explicitly specified. These expressions are:
Additionally, the fixedlist expression declares the type of item in the List returned within its listof statement.