Ada Programming/Attributes/'Null Parameter

Description
A reference  denotes an imaginary object of type or subtype   allocated at machine address zero. The attribute is allowed only as the default expression of a formal parameter, or as an actual expression of a subprogram call. In either case, the subprogram must be imported.

The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default).

This capability is needed to specify that a zero address should be passed for a record or other composite object passed by reference. There is no way of indicating this without the  attribute.