A French term, and fancy way of saying: the invoice an underwriter makes of reinsured risks.
Underwriter (insurance company).
Reinsured (insured once...then, to mitigate downside risk, other insurers buy in to the product).
Bordereau can include loss data specific to those risks, or premiums. Like...it statistically meaasures how likely (or not) it is that the tanker will. in fact, run aground in a storm over 40 knots.
Usually, this assessment and reassessment is done periodically by the company that was carrying the risk previously. New policy riders or adders are then charged to the company now being insured. (After reading all that dullness, you can see why the French term was adopted, oui?)