The system will try to detect sensitive info in documents automatically.
Here I have a document where I've put in a bunch of made up Social Security numbers, etc.
The system caught that and applied this label. If I click on it it says what was detected.
This isn't always correct, but it's there to help you be aware.
This may show up as:
"May Contain Private Info"
"May Contain Financial Info"
"May Contain Medical Info"