It's also called a head shim gasket. It goes on top of the piston sleeve between the top of the piston sleeve and the button head. It comes in different thickness. For Sirio engines, you can buy a "Sirio Head Shim Gasket Kit", which comes with several gaskets.

You adjust the amount of shims based on % nitro you are running to give a total "head clearance" between the piston and button head when piston is at the top of the compression stroke (refered to as "top dead center" or TDC).

This is a general guide based on head clearance:

16% Nitro .36mm - .40mm
20% Nitro .40mm - .50mm
25% Nitro .50mm - .60mm
30% Nitro .60mm - .65mm