Means any evidence they didn't ask for. You can basically use it to send any evidence to strengthen your case.

@Will thanks a bunch

I don’t see any unsolicited evidence option in my USCIS account. can you pls tell me where do I find it?

@A N it's should be where your documents are uploaded

@Lorin Kell I don’t see it, may be bcz I didn’t fill my form online I did paper file. #K1

I have the same issue.. I don’t have the option in my account to add unsolicited evidence. I filed I485 in paper form