The 1.5 inch standard pump union is a universal pump union used for complete assembly of your replacement pump. Should fit any 1.5 inch end on most pumps.
The 2 inch Pump Split Nut is used as a replacement for the pump union that would need to be re-plumbed. You can cut off the old pump union and replace with this split nut. This comes with the 2 screws that screw and hold it together.