What does the text inside parentheses in @interface and @implementation directives mean?

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Those are called Categories and allow you to add further functionality to your classes.

A category allows you to add methods to an existing class—even to one for which you do not have the source. Categories are a powerful feature that allows you to extend the functionality of existing classes without subclassing. Using categories, you can also distribute the implementation of your own classes among several files. Class extensions are similar, but allow additional required APIs to be declared for a class in locations other than within the primary class @interface block.

From the Apple docs on Categories and Extensions.

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