The higher this rating the more heat they can dissipate. To limit the range of resistor values to a manageable number a preferred range only is available. 3.3M would be written as 3M3 and 1.8k as 1K8 etc. On circuit diagrams tolerance is indicated by the following letters. These can be operated by means of a knob on the control panel.
Examples are volume and brightness controls.
Preset variable resistors are internal controls which are adjusted in value by means of a screwdriver. Once adjusted, they are never touched again.