The 101.4 rules talk about APNAP order—the guiding principle that if things have to be done in an unspecified order by players, the Active Player (the one whose turn it is) goes, then the other Non-Active Players go in turn order. Active Player, Non-Active Player, APNAP. What a snappy acronym! But what about before the game begins, when there is no active player? This new rule clarifies something that other rules allude to: APNAP without an AP uses the starting player as AP and the others as NAP.
103.1 and 103.2
The game begins by shuffling up your decks and choosing who goes first. In practice, most players do these together, rolling a die in the middle of shuffling or similar, but the rules have to say which happens first, and shuffling was chosen sometime in the past. Companions make it matter which happens first, since we need a starting player before setting aside cards before shuffling. Determining the starting player always comes first now, though, if it doesn't matter, you can keep on doing whatever you were doing all along. (How often do you really want to pick a companion based only on your opponent's?)
Objects only have a controller if they're on the stack or battlefield. There were five exceptions—and now there are six. The sixth is mana abilities, which don't use the stack but are still abilities that can instruct "you" to do something.
There were eight special actions—then the companion change made nine. In between edits and revisions, the change to this number was eaten (perhaps by the number seven), but now it's back and should be here for good. The rules can count, I swear.
The "excess damage" rule is taking a tiny update to explicitly refer to lethal damage, giving Zilortha a place to stick its oversized feet.
Basri joins the list of planeswalker types! Thanks, Oketra.
Dog joins the list of creature types! Hound is removed because it ain't never caught a Rabbit.
This new rule shores up support for Panharmonicon-style effects that say an ability triggers additional times. Most importantly, it slams on the brakes to make sure Sanctum of All does not cause an infinite loop of triggers. All in all, it's really just explaining how we've already ruled this sort of thing to work.
Linked abilities can get weird when one of the abilities exiles one card and the other talks about the one exiled card, but through shenanigans like Panharmonicon, we get two exiled cards. The rules around resolving this weirdness were accidentally tripping on their own feet. They said that talking about "a" card saw all of the cards if we were getting information on them or taking actions on them, but that's not true—every ability taking actions on the cards wanted you to pick one. This rule's been updated to say so. Abilities that ask for information about the cards still check both.
If something exiles something else "until" an event and that event happens before thing two is exiled, thing two doesn't get exiled at all. But Palace Jailer exiles a creature until someone else becomes the monarch—and a player may have become the monarch much earlier in the game! This rule now checks only for that "until" event after the spell or ability is put onto the stack or triggers.
Layer 1a is where we apply effects that set copiable values. It's been missing one thing from the old rules on copiable values: replacement effects that define values, like Primal Clay! Now that's here again.
This rule was missing the word "time." I wouldn't even comment on correcting this typo except that by fixing it in M21, I feel compelled to blame Teferi.
Mill triumphantly joins the list of evergreen keyword actions! The other 30 keyword actions after it all get bumped down a number. Meanwhile, in other rules, the dredge keyword is updated to just use "mill" instead of a lot of words.
The list of state-based actions keeps getting longer and longer. We were going to hit 704.5y with this update, and rather than figure out what would happen two additions later (704.5aa was the most likely candidate), I cut that knot right down the middle. All the state-based actions that only apply in specific variants are broken out into a new 704.6 rule!
Wow, look at this fresh, clean room. It smells like paint. I shouldn't spend much time here, but before I go, here's a new Commander state-based action. If your commander moves to a graveyard or exile, you get one chance to move it to the command zone. The old replacement effect no longer applies to dying or getting exiled, so your commander can die a bunch now. 903.9 down in the Commander section has also been updated with this change.
If you've got a face-down morph creature, you have to reveal it if it leaves the battlefield or if you lose. This rule has an update for what happens when you have a face-up creature that has a face-down card floating around inside of it thanks to a mutating creature spell: you still reveal that face-down component.
This rule explains how chapter symbols use Roman numerals. It helpfully provides examples of I, II, and III (better known to this audience as 1, 2, and 3). Some people argued that this rule also instructed the ancient Romans on how to count, and without stating that IV is 4, Rome would surely fall and certain new Saga cards couldn't work. To avoid dooming the modern world with a temporal paradox (stop it, Teferi), the rule has been expanded to include "and so on," clarifying that Romans and Sagas can count past three.
Yawgmoth's Will will exile cards that would be put into your graveyard. If you have a merged permanent, it could have some cards and tokens mingling. This new rule clarifies how Yawgmoth feels about this: if the permanent isn't a token, it all gets exiled, even token components; and if it is, the permanent dies but the cards among it get exiled.
When we talk about a permanent, an Elf, a Chandra, or a land, we always mean an object on the battlefield matching that description. When we want to refer to a spell, we use the word "spell" in there. That's all well-established, but commanders make it weird: we don't use "spell" to refer to "cast your commander," but if we don't, does that mean you control your commander while it's on the stack? This new rule helps to clear up the language on what "your commander" means in the various cases.