A modifier resolved for a specific checkout — the input the pricing pipeline applies. Eligibility (scope, stock, codes) is decided upstream; by the time a spec reaches pricing it is known to apply. value is the signed calc value (see modifierDelta); listingIds scopes which items it is charged on (null = the whole order); quantity is how many the buyer took (1 for an automatic or code modifier, more for an opt-in add-on).