A receipt for a liability you have handed off
Treat it as a receipt, except that what you are being given a receipt for is not money. It is the end of an obligation. For as long as your name sat in the register you owed road tax on that car, the law required you to keep it insured, and any fine it collected was addressed to you. The vrijwaringsbewijs pins down the minute all of that stopped, and a pinned down minute is precisely what you need when somebody later disagrees about it.
The RDW prescribes no particular layout. One counter prints a till receipt, another a sheet of A4, and a trade buyer may send a PDF by email; all three count. Three things have to be legible on whatever you are handed. The plate. The transaction code. A date with a time attached to it. Those are the facts an insurer, the Belastingdienst or whoever sends you a bill dated after the sale will ask for, and the document is worth little to you with any one of them missing.
Three events that produce one
Selling, scrapping and exporting each generate a vrijwaringsbewijs, and the route it travels to reach you is different in each case. In a private sale it never comes to you directly. The buyer is the one who triggers the transfer, so the register issues the document to them: a download link if they worked online, a printed sheet if they went to a desk. They then have to pass it along, and that handoff is the weak point of the entire sale. Both routes are laid out step by step under transferring a car to the buyer.
The other two events take the buyer out of the chain. Sell to a business the RDW has recognised and their own staff process the transfer, giving you the document once you have handed over the complete set of registration papers. Export the car and the vrijwaringsbewijs comes bundled with the rest of the export file. Where that sits in a relocation is covered under getting a car sold before you move, and the RDW publishes the export steps in full.
Keys last, and why that order is not negotiable
The RDW's instruction to sellers is a single line: do not part with the vehicle until the proof of transfer has reached you. It can read as excessive caution, until you consider what a buyer is really proposing when they say they will sort the paperwork out tomorrow. They are proposing that you carry the tax, the insurance duty and the exposure to fines for an unspecified stretch of time, on trust, with the car already out of your sight.
Money is a separate matter, and the RDW takes no position on it, so that part is yours to negotiate however you like. The running order is where you hold real leverage. Two arrangements work. Walk into the desk together, so the transfer and the handover occupy the same five minutes. Or sit beside the buyer while they complete it online. Either way you can verify the result without asking anyone: Mijn RDW shows what is registered to you once you sign in with DigiD, and the Kentekencheck answers the same question from the plate alone.
Cars that never got a kentekencard
The card took over from paper in 2014, so a car whose owner has not changed since then may still be running on the old documents. Work out which you have before anything else, because a paper car cannot be transferred online at all. The seller gives the buyer the registration certificate along with the overschrijvingsbewijs, the transfer certificate, and the two of you go to a counter. Your vrijwaringsbewijs then comes back exactly as it would for a car on a card.
What the paper set contains depends on when the plate was issued. Plates issued from 1 June 2004 up to 1 January 2014 brought three documents: part IA, part IB and part II. Go back earlier than June 2004 and the set is part I, part II, plus a standalone transfer certificate. For plates predating 1995, that last item appears as a copy of part 3. A missing piece is an RDW problem and not a buyer problem, so raise it with them before you write a date in anybody's calendar. The sets are described more fully under the papers a sale needs.
When it never came, or you cannot find it
Two different problems with two different remedies. If you once held it and it has since gone astray, the RDW offers a free overview listing every vehicle registered to you across the past nine years. Request it on the RDW site using DigiD, and a link to the file follows by email. Plenty of insurers will take that overview in place of the real thing, according to the RDW, which covers most ordinary purposes such as closing a policy. Put the question to your own insurer before leaning on it.
If no proof ever reached you after a sale, a scrapping or an export, that is the other case, and it comes with a deadline. Tell the RDW inside three months of the transfer and they will issue you a replacement. The RDW keeps a page for documents that failed to arrive, with the form on it. Three months goes quickly, so count from the date of the transfer and not from the day you noticed the gap.
The one thing it will not do on its own
Nobody tells your insurer. The RDW passes the transfer to the Belastingdienst, so the tax side resolves itself, but the policy runs on until you send a copy of the vrijwaringsbewijs and ask for it to be closed or moved across to your next car. Do that in the same week, because the premium keeps being collected until you do. The remaining loose ends are listed under what to settle after a sale. Keep the original somewhere findable for as long as a bill about that car could still plausibly arrive.