Are there guidelines for what code goes into com.myproject.api vs com.myproject.api.impl?

I've seen this package structure pattern a number of times. Most recently, I've been going through the Last.fm An开发者_如何学Cdroid app code and would like to understand why some code is in fm.last.api versus fm.last.api.impl. https://github.com/c99koder/lastfm-android/

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The package fm.last.apicontains the interfaces that define the public interface. In the package fm.last.api.impl the actual implementation of this interface is located.

This is a rarely widely used approach to differentiate various concerns.