What does DPR mean on a Scalextric car?
DPR stands for Digital Plug Ready. A DPR Scalextric car has a socket in the chassis that accepts a plug-in digital chip, so it can be converted for Scalextric Digital racing without soldering.
Which digital plug a DPR car needs
Most DPR cars take the C8515 EasyFit Digital Plug. DPR Formula 1 cars are the exception: Scalextric lists the C8516 Formula 1 Digital Plug for F1 cars with 2008 or later liveries.
If your car is not DPR
Older and non-DPR cars have no plug socket. Scalextric lists the C7005 Digital Conversion Chip as the retro-fit chip for converting those cars to Scalextric Digital.
Which systems the conversion works with
All three chips are listed for Scalextric Digital (SSD) and Scalextric ARC Pro. Individual car pages on SlotFits show which chip each verified car requires — for example Knight Rider KARR (C4296).