见：http://developer.apple.com/library/ios/#documentation/General/Conceptual/ApplicationDevelopmentOverview/ConfigureYourProject/ConfigureYourProject.html#//apple_ref/doc/uid/TP40011186-CH6-SW8

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An app ID is a string containing two parts,a bundle seed ID and a bundle ID search string,with a period (.) separating the two parts. Each part of an app ID has different and important uses for your app.

The bundle seed ID is a unique 10-character string generated by Apple. The bundle seed ID is what associates an app ID with a specific team. Apps that share the same bundle seed ID can also share keychain data,such as user names and passwords. Each development team is assigned a specific seed ID,known as the team ID. You should almost always use the team ID in any new apps you create; if you are doing Mac OS X development,using the team ID is your only option for the bundle seed ID,and it is provided automatically by the tools. # Note: If you have been already been developing apps for iOS,you may have prevIoUsly created bundle seed IDs for apps you have already distributed. The ability to create new seed IDs has been removed from the tools,but you may use a prevIoUsly created bundle seed ID instead of using your team ID. Do this only if you need the new app to share an existing app’s keychain data. Otherwise,use the team ID. (…)