The Unique Superannuation Identifier (USI) is used in SuperStream to uniquely identify an APRA fund and/or its superannuation product which an employee (member) is contributing to.
These are kind of like a superannuation product code, required in Lightning Payroll to be able to send SuperStream messages from the program. Your employees will need to provide these USIs to you along with their member information if they are with a typical APRA super fund. They can get a USI by contacting their super fund.
USIs are best entered into the program under Settings >> Super Fund Providers >> Create Super Fund (Online Search).