Not UA but I did a little test today which is helping me piece together the picture...

Have a trip on EVA: TPE-YYZ-YOW-YYZ-TPE. EVA is on Elite (Preminum Economy). On the AC website, it reflects 2 pieces at 28 KG per piece - which is EVA's rule. This matches my understanding of the IATA MSC.

Once a *G number is inserted, the AC confirmation e-mail shows another section titled "Additional Baggage Allowance for Air Canada Altitude and Star Alliance members" which allows 1 additional for 23 KG.

So, I am GUESSING that the eticket carries the allowance (like the old paper tickets) which allows other carriers on the ticket to process the allowance correctly.

Then, if one has status with the airlines, the allowance is really not changed on the ticket - it is really based on status and that airline's rule.

Now, I am still confused in the case with UA, for example TPAC, where free baggage allowance goes from 1 piece 50 lbs to 3 pieces 70 lbs for Gold and above.

If one does SFO-NRT-PEK in Y on UA/CA then return, how would the eticket allowance be populated by UA? 1@50 or 3@70? I am guessing 1@50 and *G allows another one. So, CA would see 1@50 + Gold 1@50 = 2@50? So, 3@70 outbound becomes 2@50 on the return?