Why is referring to outside notes illegal?
Magic is a skill-testing game. Players may prepare for a tournament by reading articles, testing with other players, etc. It is this knowledge, alongside other skills, that is being tested when a match starts; allowing outside notes would alter this philosophy. Ultimately, Magic is in your mind, not in a sheet of paper.
Most of you are familiar with "coaching." Outside notes can be seen as having a little piece of paper coach you while you play a match.
Are notes written down on pieces of paper the only ones considered outside notes?
No. Any written information not taken during your current match, be it on the face side of your cards, on your PDA, or even in the palm of your hands, is deemed to be outside notes. Even small markings such as "SB" on a card's face are not legal.
That means I can take notes during my current match, right?
Memory is not a skill being tested by Magic. That's why players are allowed to take in-game notes, and consult them during the remainder of the match. Keep in mind, though, that these notes have to be written down in a timely fashion. These notes can only be used by you, and only during the same match. You may also ask for the current Oracle wording on a particular card.
You've talked about notes in the middle of a match. What about between rounds?
Between rounds, you may consult any notes; as long as they're not illegal (e.g. notes concerning what other players picked during a draft are not acceptable). You can check your decklist in order to de-sideboard, as well.
Any other examples of what is considered outside notes?
Some of the most common practices of using outside notes are the following:
1) Sideboard plans: what to sideboard in and out.
Many players spend a lot of time testing and practicing with their decks, and develop a strong knowledge of how to sideboard against particular archetypes. Sideboarding is an integral part of Magic tournament play, and it's your skill that's being tested. Referring to outside notes indicates it's not your skill, but somebody else's. You may consult such notes between rounds, but as soon as the round starts, they become outside notes.
2) Information regarding opponent's decks.
While you can certainly get information regarding possible opponents between rounds from your friends, etc, you can't refer to these notes once the round starts.
3) Information regarding the draft or deck construction portion of a tournament.
It is illegal for players and spectators to use any notes taken during a draft, or the deck construction portion of a Limited tournament, during a match. Likewise, it is illegal for players to receive or to give notes taken during this time. A player's card pool is supposed to be non-public information during play.
To sum it up:
You may take notes about any action that occurs in the game and consult these notes later in the match. The notes should be taken in a timely fashion, which means you will not be able to write down the entire contents of the opponent's deck or take any notes that would require significant time to take. Players who take longer than is reasonably required to complete game actions are engaging in slow play: while taking notes you are not completing game actions.
Notes taken in one match may not be used in another match by you or any other. In any other match, they become outside notes, and they're equal to receiving outside assistance. To put it simply - when a new match begins, start with a clean sheet of paper.
Carlos Ho, Panama (currently at Madrid, Spain), L3
carloshod [at] gmail [dot] com