What does "add-on" no-fault laws mean?

"add-on" no-fault laws meaning in Insurance Dictionary

Statutes appropriate in certain states that enable the addition of injury defense (PIP) coverages to automobile insurance policies without restricting an injured party's right to sue in tort. In some of these "add-on" says, acquisition of these accidental injury (PI), or "no-fault," coverages is required. In other people, acquisition reaches the insured's choice, although insurers must provide all of them. See also accidental injury security.