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1. if the multiplicity is defined as "ONE2ONE", then attempts to add more than one object to either side of the relationship will result in an error
2. if the multiplicity is defined as "MANY2ONE", then attempts to add more than one object of the "to" class will result in an error
3. if the multiplicity is defined as "ONE2MANY", then attempts to add more than one object of the "from" class will result in an error

All relationships are mostly optional, so in the UML model:

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