Introduced on day 1
Cost of error
What each kind of mistake costs the business. Costs are often asymmetric - a denied boarding usually costs more than an empty seat - so the best decision is rarely the one that minimizes plain error.
In detail
Overbooking makes the asymmetry concrete. Too few extra seats sold leaves seats empty (lost fare). Too many denies boarding to passengers who showed up (compensation, rebooking, goodwill). If a denied boarding costs several times an empty seat, the cost-minimizing limit sits below the expected number of no-shows.
Write the costs down before choosing a method, even as rough ranges. They decide the metric, the threshold of a classifier (Day 13), the quantile a forecast should deliver (Day 26), and whether an accuracy gain is worth its running cost.
Unknown costs are a finding, not a reason to skip the step. Ask the decision owner for a range and show how the recommendation changes across it.