Here's the short version: Beginning with the Anniversary Update, version 1607,
you'll be able to link a Windows 10 digital license with a Microsoft account. This linkage occurs automatically if you're signed in with a Microsoft account when you upgrade to version 1607
For anyone else, including those with local or domain accounts, this step is optional
. In any case, it applies only to those who have a Windows 10 digital license. That group consists primarily of those who took advantage of the year-long free upgrade offer that ends on July 29, 2016.