The results of the party's choices and the overall storyline for the game are determined by the Dungeon Master (DM) according to the rules of the game and the DM's interpretation of those rules.[20] The DM selects and describes the various non-player characters (NPCs), the party encounters, the settings in which these interactions occur, and the outcomes of those encounters based on the players' choices and actions.[17][21] Encounters often take the form of battles with 'monsters' – a generic term used in D&D to describe potentially hostile beings such as animals, aberrant beings or mythical creatures. The game's extensive rules – which cover diverse subjects such as social interactions,[22] magic use,[23] combat,[24] and the effect of the environment on PCs[25] – help the DM to make these decisions. The DM may choose to deviate from the published rules[20] or make up new ones if he or she feels it is necessary.[26]