There's no laid down definition of when cars pass from one to the other, which leaves the question down to event organisers. In the States our friends at Historic Sportscar Racing are very happy to allow recently retired racing cars into their ranks rather than letting them fall silent. But until recently similar cars found themselves absent of purpose over here in Europe.
That was until in the last couple of years Peter Auto and the FIA's Masters series both, within a short space of time of each other, founded new series for sportscars which lived their racing lives within the last few decades.
Here at the Spa Classic that means an incredibly varied grid has lined up as part of Endurance Racing Legends, a series which made its debut a few years back as a group of high-speed demos and has now morphed into its very own racing calendar.
For those who grew up in the 1990s and 2000s this is a heady mix – you'll find everything from prototype racers from the IMSA years to big-engined GT cars which have only just come out of the modern rotation.
Finding standouts is at first pretty obvious, but more difficult the longer you watch. Ferrari 333SP wails, Panoz Esperante growls and Aston DBR9 howls are immediately recognised. But the more you look the more you see, and suddenly the barmy Marcos LM500 is catching your pupils, two Venturis, with turbos whistling, fly past and there's even a Toyota Supra GT2 car in there.
We can't quite catch our breath.
Photography by Pete Summers and Ben Miles.