This is the 14715th most frequent German word.
nichtöffentlicher
"Nichtöffentlicher" means "non-public" or "private."
The word 'nichtöffentlichen' in this sentence is an adjective describing the noun 'Raum' (room), specifying that the room is not accessible to the public, emphasizing its exclusivity.
Die Sitzung findet heute in einem nichtöffentlichen Raum statt.
The meeting takes place in a non-public room today.
Here, 'nichtöffentlich' acts as an adverb modifying the verb 'behandelt', indicating that the handling of the report is done in a non-public manner, probably in a secret or private context.
Der Bericht enthält sensible Informationen und wird deshalb nichtöffentlich behandelt.
The report contains sensitive information and is therefore treated as confidential.
In this context, 'Nichtöffentliche' is describing the noun 'Dokumente' (documents), indicating that these documents are classified or restricted and not available for public view.