Originally Posted by
WineCountryUA
This, the most restrictive fare rule applies to the entire ticket.
This.
Also, any such segment would need to be end-on-end combinable with the other flights on the itinerary. It wouldn’t be surprising for the AC flights to increase in price due to the unrelated UA segment on the itinerary, even if this were a flight that OP actually wanted to take and hence wasn’t worried about refunding.