What is class prefix in Xcode

The class prefix is a prefix for class files Xcode generates when you create a project or class file. An example of a class prefix is the NS in Cocoa classes. If you enter a class prefix of XYZ and create an Objective-C class called MyClass, the name of the class would be the following:

XYZMyClass

You do not need to fill in a class prefix to publish an app. The main point of using class prefixes is to avoid name conflicts between the classes you write and the classes in external frameworks and libraries.

Can a class prefix be anything, or do I nedd to make it custom due to my application traits?

I don't know of any restrictions on class prefixes. I recommend not using any well-known prefixes, such as NS, UI, and CF in Apple's frameworks.