I found that it is against federal law to have a pistol upper in your possession unless you also have a pistol lower. If you do not own any stocked lowers or AR rifles, then you can possess just the pistol upper. Having a pistol upper and lowers with stocks on them means you are in possession of an SBR. SBR stands for short barreled rifle. You are in violation of federal law if you possess a SBR without a tax stamp.
There is no difference between a SBR upper and a pistol upper. If the barrel length is under 16", then the upper is either a SBR or pistol upper. If it is on a lower that has a stock, then you have a SBR. If it on a lower that has the pistol buffer tube, then you have a pistol. You can have a pistol upper and stocked lowers in your possession as long as you have a lower with a pistol buffer tube also.