The height of the unit from the bottom to the top. If the cabinet is floor standing , Architects know this measurement as 'Height Above Finished Floor Level'.
The width of the unit from the outside of the left panel to the outside of the right panel. By default doors/drawers have a clearance of 2.5mm so a cabinet with door/drawer is guaranteed to fit within the width measurement and the door/drawer can be fully opened.
The depth of the cabinet measured from the front of the carcass to the back of the carcass. This corresponds exactly with the width of the side panel. The back panel fits within the carcass space so the internal depth is reduced by the back thickness.
If overlay style doors/drawers are selected (this is the default) then the total depth occupied by the cabinet with be the depth plus the door/drawer face thickness.
All dimensions are in millimetres.