All items within Celestial Frontier have an general Item Classification. The classification is the definition of how the item interacts with parts of the game and how the attributes of the item are computed. An item's classification is part of it's Item Type, but should not be confused with the Item Type itself.
The Item Type is part of the Assembly Footprint, and determines the Item Classification, Volume, and Material Density (which in turn, when combined with the Material Type, determines the Mass).
There may be multiple Item Types with the same Item Classification.