If the multiplicity of an association is greater than one, then the set of related elements can be ordered or unordered. If no indication is given, then it is unordered (the elements form a set). Various kinds of ordering can be specified as a constraint on the association end. The declaration does not specify how the ordering is established or maintained.
You can set the ordering of the association end in the definition of the association line to one of these values:
▪unordered: The elements form an unordered set. This is the default and need not be shown explicitly.
▪ordered: The elements of the set have an ordering, but duplicates are still prohibited. This generic specification includes all kinds of ordering. If you this value, {ordered} is displayed on the diagram at the association end.
▪sorted: The elements are sorted based on their internal values. The actual sorting rule is best specified as a separate constraint.