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)