My criterion for the "unique" page is this: If you had the coin in hand (so you knew the size and fabric) and were looking at the reverse, could you identify the person on the other side before turning it over? 

I consider "type" to refer to the design and legend, not including mint control marks. Additional details (such as a letter in the field or mintmark in exergue) distinguish a "variety." 

There is a fine line between these concepts and knowledgeable experts can differ on where that line is. In his book Roman Bronze Coins: From Paganism to Christianity, 294-364 A.D. Tory Failmezger listed all the types of the period. (I highly recommend that book.) We consulted about what was a different type and what was not. For example, the soldier-spearing-fallen-horseman begins large and shrinks to much smaller. Are the smaller ones a new type? If so, which size marks the transition? It has the horseman in distinguishable positions. Do those make "types" or "varieties"? There is no "right answer." Licinius issued a very common type with legend IOVI CONSERVATORI. The same design comes with legend IOVI CONSERVATORI AVGG and IOVI CONSERVATORI AVGG NN. Some are 24 mm and others smaller down to 18 mm. Some have radiate crowns and some are laureate. There are many lines that could be drawn to distinguish between them. But, if you start with a basic list and you draw fine lines pretty soon you have RIC's exhaustive listing, which was not his goal. 

Are two similar coins different types or not? It depends upon who is doing the collecting. If you are the British Museum, you want every variety of every detail. If you are on a limited budget, you probably think a different emperor makes for a different type, but perhaps a different mintmark does not. 

I repeat, my criterion for the "unique" page is this: If you had the coin in hand (so you knew the size and fabric) and were looking at the reverse, could you identify the person on the other side before turning it over?

http://augustuscoins.com/ed/unique/unique.html