Hi, thanks for the question. Was probably not evident in the article. The R.id.content_frame is one of the ids of you app. For example, imagine that your app, has a main activity, that sets as content view a layout, that is basically a frame layout container, with id content_frame. That same ID can be just reused by your SingleActivityFragment.
As you would be replacing your main activity with the single activity when testing, you can just re-used the id without any problem.