Hello
It is a digital license at the same time, so your Microsoft account serves as the license.
Very important - when you transfer license from device to device, with digital license, you do NOT want to remove the license from the old device. You also do not want to remove the old device from your Msft account. You want to transfer the license first before you remove old license.
To transfer, go to Settings > System > Activation, run the troubleshooter, choose the hardware change option. When it asks you where it is installed on right now, you should choose the old device name. Then click on Activate button. And on the next screen when it asks you where to transfer it to, choose your new device name.
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So, you may not need to use the actual 5x5 key much, but in the case you accidentally remove the old device before transfering the license to the new device, this key will come to rescue. So you do want to keep it safe. You won't find it anywhere else in your account, or in your installed Windows. So yes, it is your responsbility to keep it safe.