614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.

614.12, 614.16d

This one's weird, so buckle up. This rule handles how we process enters-the-battlefield replacements. It started out simple, but grew exceptions over time. It led to natural results in many cases, but one kept causing plenty of confusion: how continuous effects interacted with the replacement effects.

Blood Moon and Dimir Aqueduct together illustrate the confusion nicely. This rule said that you ignore Blood Moon to determine that Dimir Aqueduct entered tapped, but then it loses all of its abilities and doesn't trigger. Players learned that specific interaction since Blood Moon saw plenty of play, but then every time an identical situation would come up with new cards, they had the exact same confusion. That's a good sign that players are learning an interaction rather than a rules system. That isn't necessarily an awful thing, but it starts to get problematic when that interaction isn't an edge case anymore.

Arcane Adaptation and Metallic Mimic make things weirder, and they're in Standard together pointing to tribal happiness. If your Metallic Mimic is fond of Ferrets (and come on, who wouldn't be?), you have Arcane Adaptation making everything you own into a Ferret (okay, that must smell awful), and then you cast a Jellyfish, you have a Jellyfish Ferret spell on the stack. You control a Jellyfish Ferret when it enters the battlefield. But it wouldn't get a counter from Metallic Mimic because of how this rule worked.

So this rule's changing. Now, to determine what replacement effects will apply to a permanent entering the battlefield, you look at continuous effects that modify that permanent's characteristics coming from the permanent itself along with those that already exist and will affect that permanent. Dimir Aqueduct enters the battlefield untapped under Blood Moon. Humility causes creatures with modular to enter without their +1/+1 counters. Metallic Mimic can distribute +1/+1 counters happily to your army of Ferrets under Arcane Adaptation.

614.16d, the parallel rule about "can't" effects, also gets to go along for the ride, even though this rule hadn't hit the "no longer an edge case" threshold yet. It also got a +1 and became 614.17d when you see the new document.

Is this new system perfect? Probably not. But it results in players being able to guess an answer that matches the correct answer more often.

614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.)
Such effects may come from the permanent itself if they affect only that permanent (as opposed to a
general subset of permanents that includes it). They may also come from other sources. To
determine which replacement effects apply and how they apply, check the characteristics of the
permanent as it would exist on the battlefield, taking into account replacement effects that have
already modified how it enters the battlefield (see rule 616.1), continuous effects from the
permanent’s own static abilities that would apply to it once it’s on the battlefield, and continuous
effects that already exist and would apply to the permanent.

Clones no longer clone things on etb.