In a meeting an Italian told a puzzled English audience: "It's another pair of sleeves".
It's an Italian way of saying: "it's another thing", or "this new argument is something different or off topic". The phrase could be used to remark that the new argument is something different or off topic. I don't think it can be used for pointing something unique.
In the context of the meeting it was used to note that the new argument was about a completely different thing with no connection with the previous one. The expression comes from the medieval use of interchangeable sleeves in woman dress.
There is some similar colorful way of saying this meaning ?