In a "claims-made" insurance policy, the period of coverage applies to when the claims are filed rather than when the underlying events occurred. Usually "claims-made" policies also include exclusions of claims based on acts or occurrences prior to the purchase of the policy.
In malpractice coverage by doctors, the use of "claims-made" insurance policies requires doctors to purchase "tail coverage" whenever they discontinue an insurance policy. Tail coverage can be very expensive depending on the specialty.
The insurance industry is shifting towards "claims-made" policies rather than an occurrence policy.