These Sleeves, or inserts as many people call them, are to reduce the internal sizes of a particular pipe to enable a slightly smaller pipe to fit the connection, whether that connection is to a socket, bend or some other fitting - they should, of course, be solvent welded into place prior to the actual pipe connection being made....
1 1/2" = 1 1/2" pressure (48.3mm) to 40mm Waste (43mm)
2" = 2" pressure (60.3mm) to 50mm Waste (56mm)
3" = 3" pressure (88.9mm) to 82mm Waste (82mm)
4" = 4" pressure (114.3mm) to 110mm Waste (110mm)