There's actually another key element to this, that goes beyond normal bickering over what is classified.

The content in question related to Benghazi and the Libyan police arresting suspects. It was forwarded to HRC's private email server by one of HRC's top aides.

Kennedy was essentially trying to pull a fast one, by using obscure language in FOIA to mark the content as "unclassified but exempt" under federal records law - which would have helped HRC's case about having classified information on her server, AND would have prevent public release of the email.