Comprehensive Rules Changes
Some abilities that make mana promise extra little bonuses when you spend that mana, like Guildmages' Forum does. We know that triggered abilities on spending that mana like Primal Wellspring get to trigger for each mana if you use Mana Reflection to squeeze more mana out of the land. Now this rule also mentions replacement effects and continuous effects.
107.3i and j
Every time X appears on a card, it's the same value of X. Except when it isn't. We previously had one exception (an object gains an ability with an X). Now we have two: Chamber Sentry has an activated ability with X that's unrelated to the X in its mana cost. I've set up the rules here to prepare for the inevitable third exception to this rule.
Sometimes you want to put a token onto the battlefield, but an effect just says "no." Isn't it heartbreaking? To make it even sadder, this rule has been amended to include rules and effects, not just effects, that say a token can't enter the battlefield. For example, a Mimic Vat that exiled a manifested Lightning Bolt can certainly try to create a Lightning Bolt token, and then when the other rules scream in horror, this rule saves the day and no token is created.
Abilities can be one of two things: an object on the stack or a characteristic objects have. But wait! Players can also have shroud, hexproof, and protection! Okay, abilities can be three things.
114.1 and more
A number of rules about targets referenced targeting zones. No cards target zones anymore. Now only objects and players can be targets, and the rules can all change.
This rule tried to say "mana cost with no symbols" but also tried to say "no mana cost." Well, does it have no mana cost or does it have a mana cost missing symbols? For consistency, I tweaked it to match 202.1b, which just says "no mana cost." This isn't a functional change, just cleaning up some terms.
We've removed the forgettable requirement that names chosen when choosing a card name must come from the format being played. This was originally added to protect players from accidental misnamings in tournament settings (for example, "Shackles" in Modern—you meant Vedalken Shackles, not the white Aura Shackles) but tournament policy has evolved to protect players in other ways (you may recall some arguments around naming "Borborygmos" in Modern that led to this evolution) so the game rules don't need to do this anymore. The game rules are generally agnostic to the format being played, so this is a nice change.
Sometimes effects refer to "this [something]," but that thing may no longer actually be a [something]! And that's okay. It doesn't need to be a [something], we really mean "that thing we're pointing at." This is the rule that says so. In the 200s section. The "Parts of a card" section. Specifically, names. What does this have to do with names? Nothing. Nothing at all. I've kicked it down into the 700s where it belongs.
The subrules of 202.3 talk about when an object's converted mana cost isn't simply the total amount of mana in its mana cost (regardless of color). Split cards have a rule about their converted mana cost down in their own rules, but this information really should have been duplicated up here all along, and now they have a reminder rule.
Hi, Windgrace! You're a kitty! Yes, you are! And you're sitting right there in the list of planeswalker types! And Aminatou and Estrid, but they're not kitties, so they're inherently less cool.
Undergrowth is an ability word. An ability word appears in italics and has no meaning. To avoid giving ability words existential crises, we list them here in 207.2c, so welcome aboard, undergrowth.
I beefed up this rule to cover more of what it's intended to cover; if your Aura can't go where it's going to go, it doesn't go there. The same riders from 303.4g about moving from the stack are added here, and a reminder about tokens not being created (see 110.5d above) goes in, too, for good measure.
As a permanent spell resolves, it enters the battlefield as a new object. It generally has no memory of its past life as a spell, but there are some exceptions. Sometimes their abilities want to know about the spells they were (for example, "if this was kicked" triggers). The description here saying so was written a little too narrowly. As previously written, the clause about what mana was spent was almost a non sequitur, and you could very well argue that Wakening Sun's Avatar's ability couldn't tell where it was cast from.
I've modified the definition of reflexive triggers slightly to make it clear that they can catch "Do a thing. When you do, do this other thing" without having to worry about whether the word "allow" means that the first thing has to be optional.
We have a handful of rules that imply that creating a token and the token entering the battlefield are affected slightly differently with regard to replacement effects and continuous effects, but when we started looking at Divine Visitation, we all agreed it was a little hard to tell from the existing rules. This rule explicitly notes that replacement effects modifying the creation of tokens ignore continuous effects, while effects modifying how tokens enter the battlefield consider continuous effects.
If you attempt to manifest a card, but for some weird reason that's probably named Grafdigger's Cage, you can't actually put it onto the battlefield, so the card stays where it is. Nothing happens. I'm skeptical that this ever caused any actual confusion, but the rules should note that it doesn't get stuck as a 2/2 face-down blob and that really nothing happens.
Did you know that the word "surveil" was made up long after "surveillance" was already adopted into the English language? Now you do. And this is where that keyword action lives. Do with this knowledge what you will.
Cards with sunburst care about which kind of counters they get, and the kind they get depends on their card type. Type-changing effects, like March of the Machines, were intended to be ignored so that the cards can do what they're supposed to do and not just attack harder. Until recently, March of the Machines would simply be ignored anyway when applying replacement effects, and now that March of the Machines does get considered, the sunburst rules are taking a small update to keep them running the way they did before.
Some Ninjas aren't all that subtle or stealthy. They sit in the command zone and say "I'm a Ninja, look at me! I'm going to jump out and boop you if you don't block!" Okay, only one Ninja does that. This is the rule for Yuriko's commander ninjutsu ability.
The rules for convoke were unchanged for Guilds of Ravnica, with one little exception: I added this rule to explain what a "creature that convoked" a spell is.
I've seen the cables the Izzet uses to jump-start their cards. It's pretty impressive. You can't see them, but you can see the rules for their jump-start keyword here.
I've also stolen the Boros instruction manual on mentoring. You can't see that, either, but you can see the mentor keyword here. Come on, I can't go around giving you all the knowledge I have, can I?
In support of Beamsplitter Mage, this rule handles a spell being copied once and assigned a specific target.
Core Set 2019 didn't feature a new double-faced card icon, since the Planeswalker symbol and modified Planeswalker symbol were used in Origins years before, but Core Set 2019 should get a call-out here.
This is the rule about the "commander tax." I added a note that it's informally known as the "commander tax." This carries no "weight," just like these "quotation marks," but it helps us talk about it in official documentation like the Release Notes without resorting to undocumented slang.