ORIGINAL: vertical grimmace
The restrictor comes already installed. You have to remove it if you do not want it on the engine. (which is highly recommended.) Unless you are getting the version that does not come with an engine. If you already have an engine other than an Evolution, then you do not have to worry about it. I guess I just assumed you had the complete ARF with engine and radio.
ARF is just the plane, no radio or engine. RTF comes with radio and engine (OS engine, Futaba radio). I got the OS46AX separately, so there shouldn't be any needle restrictor, although one came in the hardware kit with the plane, but it's staying there.