What am I not understanding (Swamp, hollow squares)?

[anarka]

I'm on my third (and hopefully final) attempt to make it all the way through The Witness. The swamp area/Tetris pieces have broken my brain each time. I don't know what it is about them, but they are really much harder than any of the other puzzle mechanics I've encountered so far.

I'm underneath the swamp and I'm still trying to get my head around the hollow square rules. This particular puzzle is stumping me, but at the moment I'm not even trying to solve the whole thing, I'm just trying to understand why the top right corner is marked as incorrect as outlined. Can anyone provide me with insight as to what I'm not understanding about the rule here?

https://steamcommunity.com/sharedfiles/filedetails/?id=2538168311

[MASTAN]

It doesn't cut figure, it subtracts.

[anarka]

So I'm interpreting from your reply that I can't just cut off the block in the corner, I have to have the whole shape within the grid. Okay.

But now I've got a solution for the puzzle, and I still feel confused, and/or like I don't quite understand the rule on display here. Both blues squares are WITHIN the border I've drawn, so why does the puzzle not fail me for cutting off the yellow square in the bottom right corner? Are the blue squares able to subtract from outside of their enclosed area?

https://steamcommunity.com/sharedfiles/filedetails/?id=2538603101

[Pckables]

Originally posted by anarka:

Both blues squares are WITHIN the border I've drawn, so why does the puzzle not fail me for cutting off the yellow square in the bottom right corner?
Are the blue squares able to subtract from outside of their enclosed area?

Yes. You're drawing the final shape.
You wouldn't enclose parts that you want subtracted, as then they wouldn't be subtracted.
The bottom right block was subtracted, therefore it's no longer part of the final shape.
The other subtraction was a yellow block that was overlapping the T shape, thus removing the overlap and making it fit.