The unit amount a cedant agrees to keep on a single risk, and the unit in which surplus treaty capacity is measured.
An underwriting line is the amount an insurer agrees to keep for its own account on a single risk, before any cession. It serves two purposes at once: it caps the net exposure on each policy, and it provides the unit of account for surplus share treaties, whose capacity is expressed in multiples of that amount. Lines are set by class of risk and rarely by a single figure: a well run portfolio carries a line table that varies retention by occupancy class, fire protection quality or past loss experience. The problem it solves is net account volatility: too high a line exposes capital to one loss, too low a line cedes profitable premium and hands the reinsurer a margin the cedant could have kept. Choosing the line is therefore a trade-off between accepted volatility and retained margin, which the actuary informs with the distribution of sums insured and the price of the reinsurance on offer.
A regional mutual revises its line table in 2026 after three years of deteriorating net results. It cuts its industrial fire line from 1.5 million to 800,000 euros and raises its household fire line from 400,000 to 600,000 euros. The first decision cuts unit net exposure on industrial risks by 47 percent; the second lets it keep roughly 900,000 euros of additional premium a year on a segment whose loss ratio has run near 48 percent for five years.
Plein, Line, Plein de rétention, Ligne de souscription