在官网（网页链接）看到这则解释：
Carnival Corporation and Carnival plc operate a dual listed company, whereby the businesses of Carnival Corporation and Carnival plc are combined and they function as a single economic entity through contractual agreements between separate legal entities. Shareholders of both Carnival Corporation and Carnival plc have the same economic and voting interest but their shares are listed on different stock exchanges and are not fungible. Carnival Corporation common stock is traded on the New York Stock Exchange under the symbol CCL. Carnival plc is traded on the London Stock Exchange under the symbol CCL and as an ADS on the New York Stock Exchange under the symbol CUK. Carnival is the only company in the world to be included in both the S&P 500 index in the US and the FTSE 100 index in the UK.