What is multiplicity in UML?

Answer 1

Multiplicity defines range of number of instances of described element with some options. For example,

[3..5] - 3 to 5 instances

[0..1] - no instances or one instance

[1..*] - at least one instance

Option {ordered} means that collection should be ordered, {unique} - each element in collection is unique, e.g.;

[1..5] {unique,ordered} - up to 5 elements in order and without duplicates.