Most racing and many high performance road brake discs are designed to be mounted on to the hub or stub axle by means of a Mounting Bell.
Mounting Bells are usually made from high grade aluminium alloy although other materials can be used.
This arrangement is much lighter than a one piece disc and bell, but more importantly allows some compliance to reduce the risk of distortion due to heat expansion of the disc.
This becomes more important the larger the disc and is considered essential above 330mm diameter.
There are essentially two methods of attaching the disc to the bell, "Bolted" and "Floating". The method to be used will depend on the particular application being considered.
Note: Disc mounting tightening torque - 14Nm (10.5lb/ft)