X next to your MP number usually denotes that the MP number had to be "forced" into the PNR. If the name on the MP account doesn't exactly match the name of the passenger in the PNR, it won't take. So, on occasion, it has to be forced into the PNR. OTOH, if it's forced in, it won't allow accrual on UA + partners, only UA flights (believe me, I've tried).

Happy Travels!
Kai

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SkyWest Airlines - your friendly regional carrier and your #1 on-time airline for February & March 2003!

My opinions do not necessarily reflect the opinions of my employer, SkyWest Airlines, or those of our major partners, United Airlines, Delta Air Lines & Continental Airlines respectively.