Activision has clarified it's unique PSN/XBL accounts (Destiny)

by Xenos , Shores of Time, Friday, May 06, 2016, 14:28 (2933 days ago) @ Cody Miller

So not sales necessarily since used copies only count once, but probably relatively close to the number of actual sales.

From Engadget article when they first used the term:

"Update: We've received a response, and Activision says that the "registered users" refers to unique PlayStation Network / Xbox Live accounts, while multiple characters per account are not factored in. That's still potentially less than an exact 1:1 sales number, but reflects an impressive amount of sales since launch. Our only questions left? Why not be more specific from the beginning -- and when is the gunsmith going to be selling rare fusion rifles?"