The purpose of the NULL option for SET SESSION CONSTRAINT is similar to the NULL option for SET ROLE, where the request removes the active role for a session.
If the session is set to NULL for a constraint in place of one or more constraint names, the assigned constraint set must be assigned to the session. If you do not have the OVERRIDE privilege, Vantage returns an error. Constraint UDFs can accept a null from data rows, but not from a user session.