Originally Posted by
Often1
"Linking" is nothing more than a note in PNR #1 identifying an onward separate ticket or PNR (or both). It has no impact of any kind on whether the passenger is connecting, which he is not. It is useful only in the context of families traveling together on separate PNR's or where it may help an agent handling a rebooking for someone not particularly fluent in the local language.
Indeed. It is a free-form field.
Whether the aircraft is the same is likewise irrelevant. The physical aircraft will be emptied, all incoming bags will be processed, either to CBP for physical delivery or, if properly tagged as I-T-I, on to the next flight. If that is the same aircraft, the bags go back to that aircraf and if not, not. As OP is not checking bags, it doesn't matter.
UA has a general practice, but no guarantee of accommodating its own passengers on transfers between tickets in IRROPS, especially if due to something within UA's control. But, it's a practice, not a policy. Maybe it's a reason to fly AA, but that's not the issue for OP.
I agree with all of this.