Objective-c - There is no group to select when creating a new cocoa class in xcode

Question

The two newly created files, .h and .m, are on top of the project. When I click on the properties of the .m file, there is no Target Membership. When I click Add Files to "xxxx" on the project, I can add it to the project, but when compiling Still an error occurred, the code was taken based on http://www.jianshu.com/p/2741.... When creating a new class, what is the difference between cocao class, Header File, and Objective-C File?