In some games, the rules and aims are totally objective. A computer could (and sometimes does) say how well each player did and whether they cheated.
Then there are games where things are more vague. Most forum games and TabletopRPGs fall here. Maybe some things are spelled out, but there can easily be disagreement over whether a character can do something, whether a post was funny enough to be worth posting or so on. These rules might be adjudicated on by a Game Master or by consensus, or it may be up to each player to interpret them for themselves. Sometimes the subjective rules are well defined, but often they include unstated things like "try to make it interesting" (arguably more objectives than rules). And then there are games where all the rules are subjective. Generally games higher on the scale place more importance on strategy while games lower on the scale place more importance on creativity. Gameplay and Story Segregation tends to occur more often higher up the scale.
When a player plays a game with subjective elements as though it were an objective game, the result may be a Munchkin.
1: Completely objective
Here, the rules and aims of everyone is completely clearcut. If this game has characters, then it is still impossible to have a Munchkin without actually cheating.
Games where there is a highly competitive followings, where successful players are renowned, people see the game as Serious Business, and every detail of strategy is carefully analysed (e.g. Chess, Go, Football) tend to fall in this category.
- Almost all computer games are here, in order to allow the computer to enforce the rules.
- Note that while videogames are completely objective in terms of rule enforcement, the actual rules themselves are hidden behind the programming. This leads to things like Good Bad Bugs, where the actual rules enforced are not quite what the player might think or the instruction manual might say.
- Chess is well known to be here.
- Most sports
- Many, many other board games, card games and so on.
2: Objective rules with significant subjective strategy
In these games, the rules are almost all clearcut but more subjective judgements are important in the strategy. Generally figuring out what people are thinking and how to get other players to think something is important in these games.
- Charades: It's clear what everyone is allowed to do (aside from borderline cases of what counts as an item of media), but the importance of working out how to communicate things to other players and what other players are trying to communicate necessitate a lot of subjective judgment. The principle applies in other modalities as well, such as drawing or defining while avoiding a specific word.
- Absolute Balderdash: Apart from one subjective rule which rarely comes into play (extra points for a fake answer "very similar to" the real one), but working out answers which will trick other players and working out which answers are the real ones involves subjective thinking.
- Apples to Apples
3: Mix of objective and subjective elements:
Here, there is plenty of objective stuff, but also a lot of subjective stuff. The latter is likely to be more important, but the former is still something the players will need to worry about. If there are characters, there can be munchkins. Often there is a Game Master to adjudicate on the subjective stuff.
- D20 Tabletop RPGs
- Some theater sports, for example Typewriter.
4: Subjective play with a few objective rules
Here, the objective rules don't present much of a challenge, though technically can be broken. Dealing with the subjective ones are much more important. Here and below, a munchkin will be pretty much invincible, except by other players assuming similar techniques.
- Corrupt A Wish: What counts as a corruption is subjective, but you must post a wish.
- Scene Three Ways: There must be three scenes. The other rules, while well defined, are subjective (what counts as Film Noir, for example?).
5: Structured but totally subjective
Here, pretty much all the rules are subjective- it might be impossible to objectively break them. Something can still not really fit, though.
- Some forum games, such as ITT: we are [insert character trope] on the TV Tropes fora.
6: Unstructured subjective
These are the games which don't really have rules, so much as stories. The sort of games kids are more likely to play than adults.
- Calvinball is a fictional game in this category.