It's more complicated.

This feature means that the database knows that NULL != NULL (that's the semantic of the SQL standard), applies this rule when all fields in an unique constraint are nullable, but fails to apply it when only some of the fields in the constraint are nullable. That's an Oracle-specific bug.

Then it was repurposed for MSSQL which always considers that NULL == NULL in unique constraints. That's incorrect, and that's why I've split it in two and changed the name altogether.