"Example: My wife is in surgery, and has told me how surgical tools often come billed as a set. Rather than being able to bill for a single tool, if an operation requires another duplicate of the same tool the surgeon will often just open an entire new set." That could be because the instrument required has been sterilised as part of a set and is not sterilised individually. This is a common problem in the UK where I work (as a surgeon), - the logistics of individually tracking all single packaged instruments would be unworkable and hugely costly.So they are grouped as a set for a particular operation.