Originally Posted by
findark
Segments are on the PNR, not the ticket, and are local to that carrier's PNR.
The ticket (with coupons) is a single document shared between carriers, and the mismatch between coupons and PNR segments is what usually causes checkin issues. UA can remove a segment from its PNR without it affecting the NH PNR, and if the segment was changed without reissue it looks like they tried to apply the same coupon to the new flight (revalidation) which is always a little bit iffy with partner systems. Depending on what backend they use, some combination of carriers do a better job of pushing PNR changes to each other than others.
All in all, not surprising outcome although I'm not familiar with NH in particular.
Oh I didn't realize segments were local to the carrier's PNR. That's helpful to know. FWIW, the ticket
was reissued by United during the original code change from UA7950 to NH8, just not again at airport checkin. I guess UA didn't or couldn't push the complete PNR update to NH. (Though I'll admit I don't really understand how NH's system got the addition of NH8 but not the removal of UA7950.)
Given that PNRs are local to carriers, will phone agents typically be willing to modify their carrier's PNR to match the coupons, even if they aren't the ticket issuer?