I take it you meant the Rising Rate Fuel Pressure Regulator (RRFPR)? If so, it's used to increase the fuel pressure based on the amount of boost raised. Many are adjustable, so you can fine tune it to match the correct pressure. As for the question about whether you need fuel injectors or not, it will depend on what are you using it for? An aftermarket turbo kit but low boost shouldn't require the replacement of factory injectors; however, if it is used for higher boost applications, there is a good chance that EMS will also be needed for fine tuning purpose.
Since you didn't mention what is your plan, this is the best answer I can tell you.
Good luck on whatever you plan to do