What is the "Max markup over cheapest" rule for flights?

A dynamic rule: a fare is within policy if the whole trip costs no more than the cheapest possible trip for the same search plus X, where X is an amount in the policy currency or a percentage (up to 99.9%). For a round trip or multi-city trip, the cheapest trip is the sum of the cheapest option on each route, and prices include the service fee shown on screen. Example: a markup of €200 with the cheapest trip at €100 — trips up to €300 are within policy; from €300.01 — violations. The rule is checked in search results and again when the traveller switches fare in the basket. An alternative to a fixed limit that follows the actual market price.