I will lay out my understanding in this thread.
First, intel officials were initially barred under our constitution and laws from capturing innocent Americans on wiretaps and surveillance of foreign officials and terrorists.

Then, they said, “Well, we need to sometimes capture those innocent U.S. citizens ‘incidentally,’ but we won’t store their information. We’ll protect their American rights.”

Then, they said, “Well, we will store the info for just a little while, but we will carefully control access and we will ‘mask’ or hide the identities of the U.S. citizens so they cannot be used for political or nefarious purposes.”

Then, they said, “Well, we need to store the names & info of the innocent U.S. citizens longer than we first said. And it will go in a database. But don’t worry, in the very rare instance when names are ‘unmasked’, it will be a careful, contolled process only a few can do.”

They added, “Nobody will be able to ‘unmask’ simply because they are curious or suspicious; there will have to be extremely important and well documented reasons directly related to national security. This will be extremely rare.”

Now, some are saying: “What’s wrong with unmaskings? There are thousands and thousands of them every year…”

(I was told back in 2015 that this process was increasingly being abused by bad actors.)