There are various ways players can gain advantage by changing the contents of their deck during the course of a Magic tournament.
In a Constructed tournament, a savvy player might observe a different metagame at a tournament and use that information to change their sideboard, or move advantageous sideboard cards to their main deck. Similarly, in a Limited tournament, players can add extra cards to their deck from outside their pool to improve their deck, or modify their main deck configuration based on new information.
In order to prevent these behaviors, decklists are required at Competitive Rules Enforcement Level and higher. Having each player in a tournament register the contents of their deck allows for the enforcement of tournament rules regarding decklists, and ultimately keeps competitive tournaments fair.
This Judge Academy module will make sure you are familiar with the how’s and why’s of using decklists for Magic events!