If you go to buy an A330 or A340 Airbus will give you a "Standard Specfication" document that defines the baseline model of whatever you want. Other documents will list all the options available etc. These Standard Specification documents are customised for each model eg A330-300, A330-200, A340-200, A340-300, so from the point of view of defining exactly what you are buying they are all separate specification documents.
However, in design, engineering, construction and operational terms there is a great deal of commonality. With the original A330/340 models Airbus basically set out to build a long range 4 engined aircraft and a medium range twin utilising the same airframe (as much as possible), same tooling and assembly line, same cockpit, same systems and so on.