The Windows 10 Digital Entitlement is tied to the motherboard. This is well documented.
I do not know, nor do I particularly care, what unique identifier they are able to grab to make that link, but it's got to be there and it's got to be universally "grab-able" and unique.
You can swap out hard drives, optical drives, keyboards, you name it to your heart's content. As long as "the heart of the computer" remains unchanged you can install Windows 10 daily, if you do desire. People have posted on these forums repeatedly about changing out HDDs for SSDs on existing systems with Windows 10 and were able to clone their existing systems over to the new storage device and have everything working fine.
Microsoft wisely used as key a specific piece of hardware that very, very seldom ever gets changed on the vast majority of computers.
Brian AKA Bri the Tech Guy -> (my website address is in my profile, if interested)
In a modern society where everyone thinks their opinion deserves to be heard nothing annoys me more than individuals who mistake their personal preferences for fact.
~ Commenter "TheCruyffGurn" on The Guardian, 8/13/2014