List of already used objective-c Prefixes

https://cocoadev.github.io/ChooseYourOwnPrefix/ is probably your best bet. It's not "official", but it's a place a lot of devs would look.

In practice, as long as you don't use one that Apple uses, you'll probably be fine.

According to the Apple "Programming in Objective C" document of 2012-12-13, Apple reserves all 2 character prefixes for use in their frameworks. Users are encouraged to employ 3 character prefixes when naming their classes to avoid conflicts with Apple. See the "Conventions" section for details.

As an alternative to initial-letters prefixes:

• If you're writing an application, use no prefix. You're unlikely to encounter another class named AppController in your app.
• Otherwise, use the product name as the prefix. For example, if you've written a color picker named “Whizbang”, name its principal class “WhizbangColorPicker”.