Whenever you see a fraudulent signal indicating a "Singleton" or a "Singlet", it means that the combination of items, payment information, etc. is likely to be fraudulent.  

Examples:

payment_method singleton:

Rather than saying that only the credit card number is likely fraudulent, we're saying that the number, gateway, and type is likely to be fraudulent as a group.

items singleton:

It means that this item or combination of items is likely targeted by fraudsters.