Some useful info found online that should clear all the confusion (if anyone reading this was confused like I was).
Iv'e made bold and added info in brackets for the 350z specific stuff:
Bulb types T10-501/W5W (350z front sidelights, number plate, and boot light) and T15-194/W16W are genrally used for sidelights, but some vehicles also use this fitment for number plate and interior lights. Bulb types T20-7440/3156 (350z front & rear indicator, reverse and fog light) and T20-7443/3157 (350z Brake & Tail light, in a single bulb) are mainly used for brake, fog, indicator and reverse light functions.
The difference between 7440/W21W & 7443/W215W is that 7443/W215W is a dual filament bulb that allows two functions from one bulb, such as a sidelight and brake light in one bulb or an indicator and parking light in one bulb. 7440/W21W does not have this function.