The areas are nicely covered in the book Game Design Workshop: A playcentric approach to create innovative games (2nd ed) and the concept was first made public in EricTodd's presentation Spore: Preproduction through prototyping (ppt) at 2006 GDC.
Follow me while introducing this great tool. To start, let's ask: What do I need to prototype?
Areas of investigation for the prototyping process
Let's say that I want to know if the story of my game is going to be interesting enough. It's easy to test that, you can telltale it to others or create a short fiction story and see if others are geting excited and engaged with it. But in video game development is so easy to drift to the complex big vision side of things. Then it is when you need to use the PrototypeX to analyze your prototyping efforts.
There are four main areas in which we can fit that question that we need to solve:
Game mechanics: the formal aspect of the game. Winning, losing, economies, etc. It sometimes involved mathematics.
Kinesthetics: The game feel. Idea is that the interface is an extension of your body into the game world. How does the whole experience feels? According to Steve Wink, game feel components are: Input, response, context, polish, metaphor and rules.
Aesthetics: Mostly talking about visual and aural aesthetic. That's visual art, music and sounds.
Technology: During preproduction you’re reducing risk and if you’re doing something new, some of that risk is technical. Prototyping technology is about getting answers to successfully implement gameplay concepts or production workflow proccesses.
There certainly more cases in which to use the PrototypeX to analyze more complex, and real, preproduction situations. That's for a future post as well as some juicy examples.
A note on the prototyping medium
What's the most unusual prototype medium you can think of? Which of the four areas would it be exploring?