They need to clarify both the name used "Realm" vs "Domain"
Just curious... clarify how? It's the same as in CK2. Realm is everything you own and everything below your titles. Domain is only what you own directly.
Realm Effect:
Player King directly owns counties A, B, C.
Your vassal Duke directly owns counties D, E, F, and barony D.1 (a barony in county D).
Duke's vassal Count directly owns county G.
There is a building in barony D.1 with a Realm effect.
Now, I think every agrees on what Domain and Realm mean. The King's Realm is counties A-G plus barony D.1 listed above. The King's Domain is counties A-C. The Duke's Realm is counties D-G and barony D.1. The Duke's Domain is counties D-F and barony D.1. The Count's Realm is county G. The Count's Domain is also county G.
Note that this is simplified and there are going to be holdings in each of those counties that are held by someone, but to keep this easy to understand, I'm limiting the example to only have the one barony holding. It still works the same no matter how many holdings, counties, etc. you add.
So far I think we would all agree with how this is laid out. Now comes the part that is confusing people - that building in barony D.1. The holder of that barony is the Duke. Because it's has a Realm effect for that Duke, it affects everything in the Duke's Realm, which includes counties D-G and barony D.1. If the building were to affect the Domain instead, it would affect counties D-F and barony D.1 because county G is not part of the Duke's Domain. In both cases, it will not affect counties A-C because those are neither the Duke's Realm nor the Duke's Domain and the Duke is the holder of the barony that has the building, so those two terms refer specifically to the Duke and not to the King.
As I mentioned, "Holder's Realm" would be a much clearer tooltip to use so you understand that it's the Duke's Domain in this example and not the King's (Player's) Domain.
EDIT: And to clarify, if the holding is held by a barony tier person (someone who isn't Count or higher), then only that person's holding(s) directly benefit from the Realm effect because their Realm will always be exactly the same as their Domain since they will never have vassals. This is usually the case for City and Temple holdings since you aren't supposed to hold those directly.
Also to clarify, if that Count listed above had a barony G.1 with a building with Realm effect, the building would affect the Count's Realm, which would then include county G and barony G.1, which would also be the Domain of the Count
(if there was only the one barony within the Count's Realm and he owned it, which as mentioned in the italics above, would not be the case, but is just here for simplification). The Realm effect would not affect the Duke's Domain or the King's Domain.