There are 2 sizes of pump union adaptors.
For 1-1/2" pumps, use the 1-1/2" to 2" to make a pump 2"
(Rarely used)
For 2" pumps, use the 2" to 2-1/2" to make a pump 2-1/2"
(Greatest used for Waterway and LX pumps when you need a 2-1/2" suction)
Screws onto an 1-1/2" wet end and creates a 2" thread. Sometimes found on Aqua Flo circ pumps.