The Certificate of Deletion, a.k.a. Certificate of Withdrawal, is nothing more than a formal declaration that a vessel has been removed from the ships registry of a flag state. Opposed to people, who can have dual citizenship, or in some cases even more than one passport, vessels can only have one nationality. It is therefore not permitted to have a vessel registered in more than one ships registrar at the same time, in principle.
When registering a vessel in the ships registrar of a flag state, there are a number of requirements that have to be met. The requirements differ from ships registrar to ships registrar.
Some flag states ask for a CE Declaration of Conformity for vessels dating after 1998, customs and VAT declarations and others do not. Some flag states ask for a survey conducted by approved classification societies, others do not.
What all ships registrars do have in common is that the owner has to prove their identity, the ownership of the vessel and that the vessel is not registered elsewhere.
To prove a vessel has been removed from a ships registrar, and therefore no longer has the nationality of the flag state, a Certificate of Registry is a requirement.
There are a number of treaties in which the usage of the Certificate of Withdrawal is outlined. One of those treaties is the Convention relative à l’immatriculation des bateaux de navigation intérieure, Genève 25-01-1965, which was signed by Germany, Austria, Belarus, Belgium, Croatia, France, Luxembourg, Montenegro, the Netherlands, Serbia and Switzerland.
It is in a treaty like this that it is stated that a Certificate of Withdrawal is in principle required in order to register the vessel in the ships registry of another flag state and that it is issued when being removed from the ships registrar.
In some countries, like the Netherlands, you are not obliged to register your <24m vessel, if not being a power boat that goes faster than 20 km/h.
So when a used vessel is exported and has to be registered in an other country, the Dutch ships registrar does not issue a Certificate of Withdrawal, because the vessel was never registered, but a Certificate of Non-Regsitry. Such a certificate confirms that the vessel was never registered in the Netherlands and is nearly always accepted by other flag states.