An appliance type is the way we describe an appliance, for example Combination Boiler. Appliance types below to Appliance Groups. For example:
Warm Air Boiler
'Boiler' is the appliance group while Combination Boiler is the Appliance Type.
To add a new Appliance Type click on General > Preferences. Click on Customer module > Appliance Groups.
You can add an appliance group here and choose that they appear as an option when raising certificates