How will your game make money? Or, if it's not designed as a commercial product, how will you cover the development costs?
Monetization may seem like an odd thing to include as part of your design, but it's important that you consider this early in the design process. There a few primary options for monetization. You should explain why you have chosen the one you have, and on what basis you expect this to be successful.
Premium pricing: this is in many ways the simplest way to sell a game. You simply set a price, and put it up for sale on Steam, the App Store, etc. There are also inherent risks with this method: players today tend to be leery of paying anything for products they don't know (if you don't know about the "penny gap" you should familiarize yourself with the concept). Games that sell for a set price now tend to make far less money, and have far fewer players, than do those that are free to play.
If you are going to set a single price for your game, you must do so carefully. $1.99, $4.99, $12.99, or $19.99 might the right price -- or exactly the wrong one. What are other games like yours selling for? How much money did they make? What gives you confidence that potential players will be willing to pay for a game that they haven't played by a designer they don't know?
Try before you buy: This has pretty much fallen out of style, having earned the alternate title "nag ware." With this model, you let players play the game, or a part of it, before paying you anything. Then if they like it, they can pay the full price. Sounds easy, right? Unfortunately, people love free stuff, and they loathe being reminded that they've gotten something for free and should really pay up now, please. Applications and games in the past used to turn off some content or features, or post reminders to pay every so often. None of these are seen as positives by potential or actual players.
Free to play (F2P): Allowing players to play your game for free without the need to pay anything unless they want to sounds counter-intuitive, but it has become the dominant method of monetizing games on PC and mobile platforms over the last decade. While only 1-2% of everyone who plays your game will pay you anything, that can be enough to create a huge hit, earning millions of dollars per day in the best case. Moreover, everyone who is playing your game for free encourages its spread, acting as great word of mouth so that others are more likely to pick up your game. The more who play, and the longer they play, the more likely your game is to be a commercial success.
However, F2P is not something you can just slap on at the end; you have to consider this carefully throughout your design, and your decisions may also have ethical aspects. Some questions to consider: do you want to sell items or abilities that provide a player with a material benefit in the game, or only cosmetic items? Do you want to make sure that anything a player can purchase is also available (however infrequently) in the game for free? Do you want to show ads in-game, either incentivized or required? And in each case, do you understand the commercial viability of your decisions (e.g., "cosmetics only" games do not tend to monetize sufficiently to be profitable). There are numerous other considerations for this business model
Other: There are other ways to pay for a game's development and hopefully make it profitable -- you may be able to secure grant funding via a government agency; create an "advergame" based on and promoting an external IP; make one that is wholly supported by in-game ads; or create a game requiring a subscription of one form or another. Each of these has its own considerations that are beyond the scope of this document.