​A videogame for programmers, where the game is shipped in incomplete state and the player needs to write additional code in order to make the game complete

For example, maybe in order to finish current level you need help of a friendly NPC companion. But initially the NPC lacks any AI code, it doesn’t do anything, sitting around like a lifeless doll. But if you write code for the NPC it can become useful, with more advanced code being more helpful

Or maybe there is very limited amount of memory available for storing character-related data. So you need to code size of variables, with bigger size of one thing leaving smaller room for other things. Like, do you want to have bigger max inventory size? Great, but you need to sacrifice something, like by using lower max level for your character