The World MasterCard is the highest level card, like the VISA Signature. In terms of the Citibank AAdvantage World card their are a few differences between Gold and Platinum. The Platinum card has a mileage cap of 100,000 vs. 60,000 for Gold. However, if you are AAdv. Gold, Plat. or Exec Plat. these limits don't apply to you.

Neither card has a pre-set spending limit.

With Platinum you get an annual summary - Gold does not. There are also a few more benes that just Plat. gets.
It's different than the standard Citibank AAdvantage card in that you have no pre-set spending limit, World cards have lost luggage insurance while standard cards don't.
Hope this answers some of your questions. If you want more details of the differences, Citi Cust. Svc. is well versed in the products and can help you.
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