What they seized
By 
A new WSJ report with more detail:
FBI agents who searched former President Donald Trump’s Mar-a-Lago home Monday removed 11 sets of classified documents, including some marked as top secret and meant to be only available in special government facilities, according to documents reviewed by The Wall Street Journal.
The Federal Bureau of Investigation agents took around 20 boxes of items, binders of photos, a handwritten note and the executive grant of clemency for Mr. Trump’s ally Roger Stone, a list of items removed from the property shows. Also included in the list was information about the “President of France,” according to the three-page list. The list is contained in a seven-page document that also includes the warrant to search the premises which was granted by a federal magistrate judge in Florida.
The list includes references to one set of documents marked as “Various classified/TS/SCI documents,” an abbreviation that refers to top-secret/sensitive compartmented information. It also says agents collected four sets of top secret documents, three sets of secret documents, and three sets of confidential documents. The list didn’t provide any more details about the substance of the documents. Mr. Trump’s lawyers argue that the former president used his authority to declassify the material before he left office. While a president has the power to declassify documents, there are federal regulations that lay out a process for doing so.
We will see as the investigation continues, but this certainly doesn’t seem like a case of some marginal cases where there were disputes over whether documents should be classified or not.