Many products have optional add-on's or mandatory items that are modelled as accessories. These addons typically have a position they are placed on the item in question and can be "on/off" or "choice of a or b or c" type. An accessory slot defines the placement of the accessory on the item and any item can have 0-any accessory slots. You need to add an item (3D model) or item's under an accessory slot which the user can choose.
Read more: VividWorks 3D Configurator Documentation