"OEM" refers to the Original Equipment Manufacturer.
It means that the keys were sold by Microsoft in bulk, and that is why the price is lower than in retail.
OEM keys can be activated only once and they are bound to the device. Any hardware change or PC formatting might deactivate the license.
Also, it might require a "phone activation" - but you do not need to worry about it, we can assist you in this procedure.
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