A receiver is just that, a receiver. It is neither a handgun or a rifle, but it is a firearm under fed law. The proper way to write it up in the bound book and transfer paperwork is "bare receiver" or "receiver only" or just plain "receiver". Some manufacturers will sell the identical receiver with the word "pistol" engraved upon it for that little extra CYA.
One of the problems with an FFL receiving a bare receiver and logging it in on his books as "rifle" or "handgun" is the excise tax issue. If he purchased a bare receiver and built it into a rifle using his parts, excise tax would be due on that firearm he built. An ATF agent seeing an entry of rifle where the manufacturer was say NDS, he is gonna know NDS only sells bare receivers. The fact he logged it out as a rifle would make the assumption he built and sold a rifle very easy.
As far as pistol receivers requiring registration, waiting periods, etc. that would fall under state and local laws. You need to decide if logging it out as a "handgun receiver" with local paperwork would be required for a pistol build.