Apple's Cocoa and Cocoa-Touch frameworks, respectively for OS X and iOS development, have many similarities. Many of the most frequently used classes, like
NSString (Mac docs, iOS docs),
NSArray (Mac docs, iOS docs) and
NSDictionary (Mac docs, iOS docs) are identical or very nearly identical between the two environments.
The tag wiki for cocoa states that it is for the Cocoa framework for OS X, and that the cocoa-touch tag should be used instead for iOS questions. However, it doesn't address which tag should be used for questions that span both frameworks.