A game engine is usually defined as the code skeleton which has been made to render everything you see and interact with on the screen. The fun stuff! Subtractive versus additive. Subtractive has the method of infinite solid. By this it means you have basically a huge block which goes on forever and you cut out the bits you want to work in. This can also be used as an additive type, in that you can cut out a large cube to create an artificial void.
Additive begins as all good things do with nothingness. This is called the void, with additive you fill the void! (and people say doctors are playing God), first you need to create a container for what you want usually a large hollow cube. Holes into the void are called leaks, as the void leaks in. Advantages and disadvantages of buying proprietary! Advantages: There is a guarantee as to what you get to work with, they usually have most bases covered, it has a market standard of capabilities, the debugging has already taken place, and finally it comes with its own built in handling for every instance, also cheaper time-wise. Disadvantages: it’s not customisable, you are stuck using what is provided by the engine so you can’t add your own physics, costs a large amount of money, requires understanding of the game engine.