Also depends on how you interpret the phrase "matching numbers". To me, for something like an MGB, Triumph TR, Healeys, etc., it simply means that the engine is the same one that came with that particular car (you can usually ascertain that engine #4536783 came with VIN# or Body# 98768576 through manufacturer records). And personally I don't think it matters all that much to me, as long as the engine is right for the car.
For something like an old Corvette, Plymouth Superbird, GTO, or pretty much any American muscle car, it means something entirely else. These cars came with all sorts of options, far more than any British or other foreign sportycar. And all of them had numbers, and were recorded on a sheet of paper or three that accompanied the car down the production line. On '63-'67 Corvettes, a copy of it is pasted to the fuel tank. On some Dodge Challengers and Plymouth Barracudas (1970-74 vintage) they can be found taped to the outside of the glovebox, hidden under the rear seat, or taped to the back of the dashboard. In any event, they list every single option that was added to that particular car. So, having found that document, you may find that the Tor Red 383, four speed, Dana'd 1970 'Cuda on Rally wheels of your dreams would have to be converted to a Tawny Gold with Gold Leaf Metallic roof, slant six (the smaller one at that), automatic, steel wheeled, bench seated grocery getter to have matching numbers.
Then there's date coding, which is something I've only seen as being truly important in Corvette restorations. All-or lots of- the ancilliaries have their date of manufacture stamped on them somewhere, and when the GM engine plant in Hamtramck was putting together the 327 for your '65 'Vette, it put a starter motor with a date that corresponds really closely to the date your engine and car was built, usually to within a month for common engines, a bit longer for something special-ordered like an alloy blocked 427. When you go to the parts department at Local Chevrolet to buy a replacement starter, it's dated for whenever it was made......possibly several years after your car was made. When the concours judges see that, you lose points.
Which is why I like British roadsters-they are usually easier to get "right"!!!
-William