Passenger details. Exactly one adult must have isHolder=true.

Deprecated: send all ancillary selections via ancillaries. Still accepted for back-compat; ancillaries takes precedence.

Unified ancillary selection input. Each item: {id, quantity, passenger_id}. Takes precedence over extraServices when both are provided. For cars, both booking-flow and pay-at-desk selections go here; Mercury routes each to the correct provider bucket by its ancillary kind, so requestServices is no longer needed.

Deprecated (cars only): pay-at-desk selections now belong in the unified ancillaries field, which Mercury routes by ancillary kind. Still accepted for back-compat.

Answers to the account's custom fields for this vertical, keyed by definition key (from GET /v1/custom-fields). Values: string for text/single_select (option key) and date (YYYY-MM-DD), list of option keys for multi_select. Answers are validated against the account's definitions; any invalid answer fails the request with a field-level 422. An empty value ("" or []) means unanswered and is stripped at the boundary, so it is treated identically to omitting the key.

Optional list of protection-plan Mercury UUIDs from GET /v1/stays/fares/{fareId}/protection-plans. UUIDs are translated to provider quote IDs before forwarding.

Optional per-room guest allocation for a multi-room stay booking. Each entry lists the passengers indices assigned to that room, mirroring StaySearchRequest.rooms from search so a client can carry its search-time room split into the booking. Omit for a single-room booking — identical wire behaviour to today. When provided, every adult in passengers must be referenced by exactly one room, and exactly one referenced passenger must have isHolder=true.