A windstorm produces four thousand claims across three days. How does your system decide that is one event for reinsurance purposes, and what turns on the answer?
The treaty defines the occurrence, usually as losses from one peril falling inside a stated window of consecutive hours, and the cedant chooses where that window starts. Grouping the claims one way reaches a higher layer once; grouping them another way pierces the retention twice, so the aggregation is a financial decision resting on loss timestamps.