R.I. Gen. Laws § 30-15.9-7: Supplementary agreements.

Inasmuch as it is probable that the pattern and detail of the machinery for mutual aid among two (2) or more states may differ from that among the states that are party hereto, this instrument contains elements of a broad base common to all states, and nothing herein contained shall preclude any state from entering into supplementary agreements with another state or affect any other agreements already in force between states. Supplementary agreements may comprehend, but shall not be limited to, provisions for evacuation and reception of injured and other persons and the exchange of medical, fire, police, public utility, reconnaissance, welfare, transportation and communications personnel, and equipment and supplies.