apple保留两个字母的前缀，建议用三个字母做前缀

In order to keep class names unique, the convention is to use prefixes on all classes. You’ll have noticed that Cocoa and Cocoa Touch class names
typically start either with 

NS

 or 

UI

.
Two-letter prefixes like these are reserved by Apple for use in framework classes. As you learn more about Cocoa and Cocoa Touch, you’ll encounter a variety of other prefixes that relate to specific frameworks...

Your own classes should use three letter prefixes. These might relate to a combination of your company name and your app name, or even a specific component within your app. As an example, if your company were called Whispering Oak, and you were developing a
game called Zebra Surprise, you might choose 

WZS

 or 

WOZ

 as your class prefix.

apple建议用三个字母做前缀