The zeebrief is the certificate of registry issued by ILT, the Dutch transport inspectorate. ILT calls it internationally recognised proof of a ship's nationality, much like a passport, giving the right to fly the Dutch flag. ILT and the Dutch government both say pleasure craft don't need one to sail at sea.
What a surveyor checks
The name, home port and tonnage on the certificate against the boat. ILT says it stays valid while those details are unchanged, and becomes invalid when the boat is sold, so a buyer planning to sail abroad under the Dutch flag needs a new one. Getting it involves a Kadaster inspector measuring the boat.
In the report
"Zeebrief seen, details matching the vessel. Note that it lapses on sale; the buyer applies to ILT for a new certificate if required."