Maybe I'm just not thinking but I don't know of any "sfi"....maybe you are thinking of Speed density ...or instead of the "s" you meant a "c" which would stand for carbed...
I think what you guys are referring to is "SEFI" which stands for "Sequential Electronic Fuel Injection. Which is what all '86-up Mustangs use. The other which you're referring to is not "SFI", but is in fact "CFI", which stands for "Central fuel injection". It consisted basically of an electronic throttle body mounted centrally on the lower intake manifold controlled by an onboard computer, that utilised fuel from 2-58lb injectors, I believe (similar to the GM set up). They were used on ‘84-'85 GT Automatics. Although they ran well in stock form, they are to be avoided from a performance standpoint like the plague.
im with gearbanger on that one (assuming he and i both are guessing at what you mean).
SEFI is usually refererred to as EFI, as far as i know. they fire, as the name suggests, sequentially. CFI fires in batches or bank to bank.