You can create a USI for a row-level security-protected table as a composite of a row-level security constraint column and the columns of a NUPI for the table. This property can be used to implement polyinstantiation.
Polyinstantiation is a property that allows a relation to contain multiple rows with the same primary key value, where the multiple instances are distinguished by their security levels, where a security level is defined by a row-level security constraint column.