什么叫回溯再保险Retrospective covers

如题，什么叫回溯再保险

从人寿再保险角度来说的话，回溯指再保合同cover的底层原保保单生效时间早入再保合同的生效时点。
比如再保合同是2020年3月签的，cover从2020/1/1到某个时点期间生效的全部直保保单。

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