Return the company-defined custom fields configured for booking checkout, grouped by vertical (flights, stays, trains, cars). All four keys are always present; an empty array means no fields are configured for that vertical. Pass vertical to trim the response to one vertical — the other three keys stay present as empty arrays, so the shape never changes. Each definition is one variant of a union discriminated on type, carrying only that type's constraints; clients should skip definitions with unknown future type values (new types are additive). Field help text is resolved for the request's Accept-Language, and date-field windows are resolved to concrete earliest/latest dates. An upstream outage returns 503; an empty 200 is never used to signal a failure.