The notation for a LiteralNull varies depending on where it is used. It often appears as the word “null.” Other notations are described elsewhere for specific uses. Source Unified Modeling Language 2.5.1
LiteralNull: In the context of a MultiplicityElement with a multiplicity lower bound of 0, this corresponds to the empty set (i.e., a set of no values). It is equivalent to specifying no values for the Element. Source Unified Modeling Language 2.5.1
A LiteralNull is intended to be used to explicitly model the lack of a value. Source Unified Modeling Language 2.5.1
