The Powerform Controls range of Speed Controllers is designed to allow the end user to adjust the fan speed easily. They have a minimum speed setting that is usually adjusted at system commissioning. The user can then adjust the speed between that minimum speed and full speed, as required.
The smaller 5A MSC1200 will fit behind a standard wallplate whilst the larger units are split into a control unit (which fits behind a wallplate) and the power electronics. This is because the electronics are too large and generate too much heat for it to be appropriately mounted in a wall cavity.