The "far right delete" removes mail accounts, sql, etc. attached to the entity you've chosen to delete - be it domain or sub-domain. I deleted a few sandboxes earlier using this Button of Doom and it seemed to leave the directories and files intact on the controlling user accounts (although I removed them manually in shell quite promptly, so it's possible the files were queued to be removed).
It pops up "zomg! serious business!" alert when you hit it to be sure you really want to follow through with the ensuing destruction - and politely reminds that a deleted domain can't be hosted on a different account if it's been purchased with a referral link or discount.
After all the Shock and Awe, it then placates you with a soothing note that you may re-add the deleted entity back to your account if you want to re-host it again sometime.
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