These are essential for spacing the pipe in a short connection kit. There are 3 sizes for 6, 8 and 10 inch pipe. the thickness is 10mm for 6 and 8 inch pipe and 12mm for 10".
For the 6 and 8 inch 2 spacers are required per end , for the den you need 4.
They may also be made of clear 10mm acrylic on occasion.