At Competitive and Professional Rules Enforcement Level during drafting, deck construction, and playing of matches, players may not use electronic devices capable of taking and storing notes, communicating with other people, or accessing the internet (with the exception of taking brief personal calls with the opponent’s permission).
The previous incarnation of this rule banned all electronic devices, but was relaxed to only those that are capable of taking and storing notes, communicating with others, or accessing the internet. This allows dumb calculators as life keepers, people can have pace makers, Brian Kibler can sit and listen to music while shuffling, etc.
At Regular Rules Enforcement Level, electronic devices are permitted, but players may not use them to access information that contains substantial strategic advice or information about an opponent’s deck.
Device use during a match other than brief personal calls must be visible to all players. Players wishing to view information privately on electronic devices during matches must request permission from a judge.
The Head Judge or Tournament Organizer of a tournament may further restrict or forbid the use of electronic devices during matches.