Making Money with Android

Difference between a paywall and simply paying for additional content?

Say for example I have a game which has 3 possible paths you can take to get through the game and you get to choose to do one for free and the other two you must pay for. Is this an example of a paywall or simply paying for additional content.

What defines a paywall vs. simply paying for some extra levels.

What is it? Timing? Psychology?