End-to-end fares allow combinations of multiple fares on the same ticket ​​​​​​. It's as if buying two separate tickets, except they are issued as a single ticket and you get protection in case of IRROPS or schedule changes. Usually the airline operating longer sector owns the ticket / ticket number and most restrictive fares rules of all included fares apply to entire ticket. Most OTAa and even Delta reservation would automatically price end-to-end fare combinations if both fare components allow end-to-end.

For some reason Delta does not enable end-to-end combinations on most fares and in the past it forced me to connect from SkyTeam partners (specifically SU or CI) to Alaska and JetBlue in JFK and LAX, where DL flights were available and plentiful but could not be used on the same ticket unless paying full Y.

I understand why Delta does not want to allow end-to-end with it's own fares when it has through fare for same markets: through fares make it easier for airline to charge market price for A-B instead of charging A-X plus X-C which is a sum of completely different markets A-X and X-C.

But for markets which are not served by Delta, it would make sense to allow connections at least to SkyTeam partners when through fares are not available.

I understand it maybe easier to disallow end-on-end combinations for all fares, but in the fare rules end-on-end rule can be customized for certain markets and certain partners. Maybe a little bit of extra work by fare/revenue department can go a long way to enable more connections and take some revenue away from Alaska or JetBlue.