If on a UA ticket but the first flight is a codeshare and/or partner operated flight, you check-in for boarding pass and bags with the operator of the first flight, not UA. This may mean going to a different terminal.
If doing online check-in, you may need the partner's PNR identifier, Record Locator / PNR of Star Partners on UA purchased Itineraries
Note -- The UA PNR should work for LH Group airlines and some others

Bag allowance can be tricky
-- UA unique elite bag waiver privileges will generally not apply.For *G elites, checking with a *A, your *G bag allowance of -- If the original or destination is in the USA, the DoT rules apply. In other cases, the IATA Most Significant Carrier rules apply.

• DoT rules state the marketing carrier of the first segment determines the bag fees for the entire itinerary. So if a UA codeshare, it is UA rules. While *A elite benefits may apply, UA unique elite privileges will not

While initially, the UA BP will not show up on the app, it will later. Once the non-UA flights have been flown / boarded (in technical terms, their segment coupons have been lifted.)


Archive: Checkin on UA ticket but first flight not UA operated? Who/Where/BPs/Bags? {Archive}