NULL Type
12-1
The NULL type is used where data in a component either must be null (that is, empty), or not accessible.
External Form -- Input
The only valid input is a null value.
External Form -- Output
A null value is always returned.
Local Form and Behavior
The component can be created by an input operation, but no contents are modified. If it already exists, no data is modified.
Named Values
Named values are not applicable to the NULL type.
Constraints
Constraints are not applicable to the NULL type.