The Type of Service field was replaced by the DiffServ field. The Class Selector behaviour provides backwards compatibility to IP Precedence.
In IP Precedence, packets with a higher value would be forwarded quicker than packets with a lower value.
The first three bits of the DSCP Field are used to identify the IP Precedence to preserve the backwards compatibility. The last three bits when set to zero identify that the DSCP per-hop behaviour is set to ‘Class Selector’.
Devices that do not support Differentiated Services and only support IP Precedence do not recognise or read the last three bits.
There are eight class selector classes between CS0 and CS7. They correspond directly with the values seen in IP Precedence.