We plan to fly JFK-CDG in business on AF flights. I'm I better off booking through AF or booking the DL codeshares?

Sorry for asking such a basic question.

Assuming the cost is the same, the main thing is how it'll credit.

The MQMs will be the same, unless you managed to book the paid "O" business class on AF (which only earns 100% MQMs and may have crediting headaches since it's also used as an award class: Air France / KLM O paid business class fare earning with Delta (Consolidated Thread) )

If you book it with DL flight numbers, you will earn MQDs based on the actual fare paid including carrier-imposed fees. RDMs will be some multiple of this number depending on your SkyMiles status.

If you book it with AF flight numbers you will earn MQDs equal to 40% times the distance flown. RDMs will be 2x distance plus potentially a multiplier depending on your status (or only 1x in paid "O" class on AF)

Which is advantageous depends which set of numbers you care about maximizing and what the distance traveled will be.

The cost would be the same.

JFK-CDG is 3,635 miles, so RT on AF would be 3,635*2*0.4=2,908. The better MQD would therefore depend on whether the fare is more or less than $2,908.

Who do I call in case of schedule changes or other issues? Given DL phone problems, that might be an advantage of booking through AF if the marketing carrier is the contact. OTOH, a DL voucher would be much better for me than an AF voucher, pushing me towards DL, if those that becomes applicable..

The general rule is that it is the ticketing carrier you call. This is not always the same as the marketing carrier.

With the DL/AF partnership, I think, but do not know, that the marketing carrier of the Atlantic leg will always be the ticketing carrier. So it would be the marketing carrier in your case. Until day of departure of course.

BTW, does anybody know what "day of departure" actually means?

You call the ticketing agent (ticketing carrier if you bought directly from an airline) until the day of travel, in which case the operating carrier causing the IRROPS is generally responsible (although the ticketing agent is also capable of helping).

Ignoring the crediting issue, the best way to book from an ease-of-travel standpoint is as much as possible to purchase directly from the airline which is both the marketing and operating carrier.

There's a bit of a gray area, but usually starting around T-48h from departure when the segments are transferred to the DCS. It's kind of rare to have IRROPS at this point, but this is also the dividing line between IRROPS and schedule changes ("my flight was canceled" versus "my flight was dropped from the schedule").

My only hesitancy regarding this course is that if our plans change, DL would issue us a DL voucher and, I presume, AF would issue an AF voucher. I'm imaging that a DL voucher would be easier for us to use, as we tend to use AF only for France and DL throughout Europe, etc. Would an AF voucher be equally useful?

AF can book DL flights, including solely domestic flights. If solely domestic you'll be purchasing a DL fare, but from AF. Basically AF/KL/DL have agreed to sell each other fares.

Itll be easier to rebook (aka online rebook) with AF ticket if new travel involves TATL travel. Otherwise you'll need to call in.

FWIW I would go with AF tight now. Beyond issue of long wait times, DL agents are so new that they don't know policies.

Here, let me ask more basic questions (I assume).
I think I understand operating carrier, but when would the marketing carrier and ticketing carrier be different?
What is DCS?

Thanks.

Broadly speaking the operating carrier is whose plane and crew it is. This is often referred to as "metal". You can think of it as, whose paint job is on the outside and whose uniforms are being worn.

Marketing carrier is whose code appears on the flight: you and I may be seated next to each other on the same aircraft but if I bought it as DL123 and you bought it as AF456 then I have a DL-marketed flight and yours is AF-marketed.

Ticketing carrier is who actually issued the ticket, also referred to as ticket stock or "plating" (because there used to be actual imprint plates involved). This can be identified by the first 3 digits of the ticket number, 006 for DL and 057 for AF. Generally, if you buy through an airline, it will be ticketed on their stock.

If you buy through a travel agency you will still end up with a ticket issued by the airline (006, 057, etc.) but you generally have to go through the agency for changes before day of departure.

There are some edge cases etc. I'm glossing over (not least because I'm not 100% sure what they all are )

I wasn't familiar with this acronym either, but from searching I assume https://en.wikipedia.org/wiki/Departure_control_system

The open question seems to be when would the ticketing carrier be different from the marketing carrier.

A carrier can issue a ticket for any airline they have an agreement with. Ie DL could if it want issue a fully UA operated and marketed domestic ticket. They normally wont as they basically become your ticket agent, but not because they can't.

Potentially any time you book a ticket that involves flights from multiple airlines. In such cases the ticketing airline (say, DL) will usually "want" to book the other airline's flights using its own DLxxx codeshare, but (1) sometimes it doesn't have a codeshare on that particular flight (examples for DL include some flights operated by AS or HA, but also some less commonly traveled foreign partner routes), or (2) specifically for DL, if you use an upgrade certificate to upgrade an AF/KL partner flight the mechanics of the process will require DL to convert the flight to an AF/KL marketed number even if you originally booked it as a DL codeshare (3) there are also manual ways to ask for, or use ITA Matrix to book on your own, a mixed airline itinerary with different marketing carriers.

As flyerCO alluded to, some airlines refuse to issue tickets that don't have at least one segment marketed by themselves, but others don't (I once had an all-DL itinerary that I couldn't get to price out on DL's website but managed to get it ticket by Air Canada lol -- I ended up cancelling it within the 24-hour risk free period though)

Yes, for (mumble mumble legacy reasons) they copy all of the reservation information into the DCS before departure (I know UA does it around midnight Chicago time the third night before departure, less familiar with DL). The DCS then takes over as the system of record for the flight. I'm sure there is some sharing on the backend, and it probably depends on the particular carrier and system, but it's the general delineation between "this is a flight we have scheduled" to "this is a flight we are operating". Schedule changes become delays, deletions become cancels, etc.

Oh, I left off the obvious and probably most common case of, when you use one airline's points to book an award flight on a partner airline, it will be ticketed by the airline whose points "currency" you're using (you are necessarily buying the ticket from them) but award flights are always booked using the operating carrier's code (I believe; or maybe just the ones I have experience with)

Theres one or two oddball cases where award gets booked as codeshare. However 99.9% its operating carrier.