Unique and Ordered Collections
Collections are created as a result of assigning multiplicity to an element. For example, if System is composed of one or more Sample (see example), I can specify that the set of elements typed by Sample is ordered. In other words, when instances of Sample are created, there is an order to them (i.e., first Sample, second Sample). The property of is assigned to the role’s of an association. In the example, the role of sample is ordered to indicate that the order of the creation of each instance of Sample must be kept track of.
Example of an ordered, non-unique collection
By default, the IsUnique property of a role is set to True, unless otherwise specified (see example). Uniqueness is only indicated on the diagram if the instances are NOT unique (see example). If a role is unique, then every instance of the Block that is created is assumed to be a different entity than the other Instances. In other words, the first Sample cannot be the second Sample.
Reference:
