Specify the Association

Define the association between the First Class (Claims in our insurance example) and the Second Class (Policies in our insurance example).

One to Many
In the insurance example, if there could be a situation where one policy could have several claims against it, you would define the association as one to many. The first property (Associated Policy) in the first class (Claims) would be single valued and the second property (Associated Claim) in the second class (Policies) would be multi-value.
Many to One
If there could be a situation in which many policies could have just a single claim, you would define the association as many to one. The first property (Associated Policy) would be multi-value and the second property (Associated Claim) would be single valued.