I am wondering what exactly is picture-in-picture.

Is that when people interrupt the movie and come up on screen and talk about it and perhaps show some behind the scenes stuff? And does it really have to be picture-in-picture (where they come up in a little video window in the corner) or can in be where they pop up full screen and stop the movie?

Another question.
When they do stop with the video portion and it goes to audio only does that become an audio commentary at that point (even though audio commentary isn't listed as a special feature)?

Oh wow. Pip is not a DVD feature Otis an old antiquated television feature which is all but gone. In appearance it resembled what you describe, it was used to watch to different things at the same time. Two football games, for example.

Risky Business (883929014897) has both PIP and Audio Commentary marked..

The Commentary is a separate Audio track and includes PIP of Tom Cruise and a couple other starting at a screen.  We get to watch them watch the same film we are watching...very silly.

But it qualifies as both.

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or can in be where they pop up full screen and stop the movie?

But if they only stop the film and do something that is not Picture IN Picture by definition.