Every liability policy needs a rule for which policy year answers a claim. The two CGL forms use different clocks. An occurrence form looks at when the injury or damage happened. A claims-made form looks at when the claim is first made.

Occurrence form: the policy is triggered by bodily injury or property damage that happens during the policy period. It responds even if the claim arrives years after the policy ended. Example: a customer is hurt in May 2024 while Insurer A's occurrence policy is in force; the suit comes in 2026 after the insured has moved to Insurer B. Insurer A's 2024 policy responds.

Claims-made form: the policy is triggered when the claim is first made during the policy period, as long as the injury happened after the retroactive date and before the policy ended. Example: with the same insurer renewing each year and a retroactive date earlier than the injury, an injury in 2024 that produces a first claim in 2026 is handled by the 2026 policy.

Why write claims-made? Some exposures have a long tail: injuries from drugs and other long-tail products, or from professional services, may surface many years after the policy ends. An occurrence insurer stays exposed to those old policy years, which makes losses hard to predict and price. A claims-made form ties coverage to when the claim is made, so the insurer can close its books sooner.