Multiplicity defines how many instances of one model element may relate to another element.
UML class diagrams show Multiplicity at association ends and attributes. Therefore, it defines whether a relationship or value is optional, required, single, or multiple.
Common values include:
- 1 means exactly one.
- 0..1 means zero or one.
- 0..* means zero or more.
- 1..* means one or more.
- n..m defines a specific range.
For example, one Customer may place zero or more Orders. Meanwhile, each Order may belong to exactly one Customer.
The notation does not define text length. For example, String [20] describes twenty values rather than twenty characters. Therefore, a separate constraint must define character limits.
Clear quantity rules reduce ambiguity and support correct data structures, business rules, and system validation.
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