Exchange at PAX level . How to use Post/ /bookings/{bookingId}/exchange-offers

[TOP-PHE]

I see the AnonymousPassengerSpecification/externalRef* as being mandatory as well as the passenger type.

In the case of a railway that does not allow to change the Pax but only the trip (like changing the hour of the trip to the next train...), having the external ref of the PAX is normal, as it will be used to calculate the new offers.......but why in this case to have passenger type mandatory as the passenger type is already defined in the Railway system and can be found with the External ref ?

In the second case of an exchange of the Pax (Same trip but a different PAX), why to have the externaref PAX as mandatory at the moment of the offer request.

I see only one explanation on my side;
The external ref element is used to identify which PAX will be exchanged in the current booking (if for example you have 5 PAX in one fullfilment (in airline it is much simpler One PAX, one fullfilment for everybody...) but it is ilogical, as it is flagged as being an info used for OfferSerarch in the way the object is structured.

Can you provide more explanation on how thhose lements are used ?