Accessory kits mainly cover 3 areas. Manufacturer specific ducting kits/pipes, microwave fitting kits and stacking kits. The ducting items are manufacturer specific ducting parts which can be used to vent air from a cooker hood to outside. The microwave fitting kits are kits which you use to build a specific model in to an available kitchen unit.
Finally the stacking kits are kits which are used to stack 2 appliances (normally a washing machine and tumble dryer) on top of each other, which is very space efficient as It reduces the amount of floorspace needed for the machines.