Yesterday I booked flights to Cusco, Peru on LA, but booked as DL codeshares with DL flight numbers. I called in to DL reservations because I specifically wanted to confirm the terms and make sure that if I needed to cancel/change my flights I would get a DL e-credit and not something usable only on LA.

The agent gave me a fare, for Latam flights in Light class but booked as DL flight numbers, and said the flights were changeable for a DL credit and miles-eligible. She then booked the flights but immediately said, "oh wait - those flights booked into DL E class and are actually basic," meaning they're non-changeable. So then she cancelled those flights and rebooked me into Main at a somewhat higher price, and now I'm in T/X fares. She claimed this was a "glitch" in the system, where flights show up as not basic until they're booked, and this is a known problem. That sounded odd, but who knows how the different airline systems work together.

Today, though, I went to Expedia to price out flights to Santiago, Chile, and there I see Latam flights in Light economy (booked as LA, not DL codeshares), and those flights are in N fare class. According to the DL website, N fares are not basic economy, and they earn DL miles.

So it seems that some Latam Light fares do book as basic economy for DL purposes, but others don't. Making things more confusing, Latam also does have a "Basic" fare class, which is cheaper and more restrictive than Light.

Does anyone know how this works? Are Light fares different depending on the country, or based on whether the ticket is issued as an LA or DL flight number? Thanks for any insight.

Light is NOT basic. They are restrictive, yes, but they are not basic economy. However, DL does not sell light fares. Light fares are sold by other airlines. If you ticket a "light fare" on DL ticket stock somehow, then yes, you can treat it like Main Cabin. However, light fares ticketed on other airline stock will have some level of restriction and most foreign carriers do NOT have the same penalty-free change policies as the US carriers do.

That doesn't seem to be true. This is a screenshot from DL's website, showing Light fares from Latam that book into E (Basic Economy) on Delta ticket stock:

Oh wow. I stand corrected!! This is a new development: before, DL could not display these fares.

Just to be clear, these are Delta Basic Economy fares, not LA Light Fares. When the flights are all DL coded like this, you are booking DL fare filings, not partner fare filings. When DL coded, the fare classes are from DL fares classes (in this case 'E' for DL BE class), not LA fare classes. If you go to the Fare Rules link on delta.com, you will find the DL fare basis codes and fare rules (which can also be looked up on ExpertFlyer under DL fare filings for JFK-CUZ). The "Light" here just designates the LA "cabin" (not really a separate cabin, of course, on BE/Light fares) on the LA operated flights. The fares under "Delta One" heading are also DL D1 fare filiings, but the "Business/PE" showing under headings just indicates the cabin on LA flights (not LA fares).

These DL BE fares have same restrictions as any other DL international BE fares with $199 "cancel" fee to receive credit from ticket for booking another ticket. DL BE fares always have a 'B' in second to last letter of fare basis codes. DL D1 fares always have 'D' in this position. The 'X' below in fare basis code indicates the coach class used to perform dual-inventory check as all BE fares are dual-inventory fares.

If you managed to plate a Light fare on DL stock, it would be treated like Main Cabin in terms of exchangeability right?

If it wasn't a DL fare (viz. it was an LA fare and thus an LA flight number), yes. The fare saying "Light" is not enough, though.

Just for an updated reference point. I booked a light fare on LATAM.com They were LATAM (LA) marketed flights, but all DL operated. They booked into a Delta X class (need to check LA fare code). I was able to select my seat assignment on DL (and since it is in an X, the operating carrier baggage should apply). If you use this method for LA marketed/only DL operated you can save a few bucks and get bags. Note I'm a DL medallion, so not sure what baggage allowance shows up for non-elite when you check-in with an LA marketed flight.

So what about domestic Light fares on LATAM...do those give seat assignments, SkyMiles and Million Miler credit to Delta Diamond Medallions? Booking Salvador-GRU, but now worried it will be so restrictive so no Delta perks.

EF shows they just book into regular LA booking classes (G, O, Q, N, etc), so they would be eligible for mileage/MQD earning and MM miles (Fare basis codes for Light fares end in a 1, Standard in a 4, and Full in a 7). It looks like LA Elites still get free seat assignments, baggage, etc. on Light fares. Looks like DL Medallions should be eligible for these too as don't see any exceptions listed for LA Light fares -- https://www.delta.com/us/en/skymiles...yteam-benefits . However, I would not expect any of these benefits to extend to non-status companions which is typical when flying on partners.

Just adding a reference point.

I booked GIG - BWI on a LATAM light fare [All LA coded with one DL operated] as a Diamond. I got free seat selection , (didn�t try bag), and on Delta.com the light fare shows in booking into a DL Q class.

If it's showing as a LA coded flight delta.com, then it's a LA Q class, not DL (and it will earn Skymiles according to LA Q class on LA partner earnings chart). LA Q class is a relatively cheap booking class while on DL it's on the higher end (YBMHQKLUTXV). Like AF/KL, LA does not have a separate booking class reserved for their "Light fares" and you can only distinguish between Light and Regular economy fare from their 8 character fare basis codes. Looks like they use the 5th position with L = "Light", E = "Plus", and D = "Full". The booking class will just be one of their Economy fare classes (and will be taken from the first letter of the fare basis code).

It doesn't look like LA shows much detail when booking on their site. You can potentially get booking classes and fare basis codes from ITA Matrix if you can price on there.

I am looking to book a partner award ticket using either DL points or Qatar Avios. The flight is operated by LATAM.
The fare class shown on Delta (and Qatar) is "T". What is this equivalent to on LATAM? Basic, Light, or Full?
I am trying to determine whether or not the fare includes seat selection and/or checked bag.
Thanks.

Award bookings on partners will be partner coded using partner booking classes no matter whose miles you use to book and T is the LA booking class for Economy awards. I would expect it to be treated like a regular full Economy class booking. As far as I'm aware, DL is pretty much the only airline that has a Basic/Light Economy award booking class (NE) for coach which can only be booked using DL Skymiles. You can't book DL Basic Economy awards on any itins that include partner metal in them and they will always be booked as Economy/Main type awards for coach cabin.