provides a great primer to the way the US system of national security classifications (Confidential, Secret, Top Secret) and access controls work.
I highly recommend downloading the document to read the section STATUTORY AUTHORITY AND DEFINITIONS paragraphs 8-23, starting on page 3.
Even if you have a clearance and have handled classified information, you may not be aware of the details described there and the impact of controlled access programs for Secure Compartmented Information (SCI).
Please do not launch into any of the current political issues surrounding this affidavit; this community forum is not the place for that discussion. I just note that the cited section is a pretty good source for an introduction to the topic of handling classified information under the US national security requirements, an essential knowledge for working information security for the US government.