Modifications are alterations made to your vehicle that change its original manufacturer's standard construction or performance - such as modified suspension.
Accessories are things that are added to your vehicle for convenience or attractiveness - such as tinted windows in a car - that don't change its original manufacturer's standard construction or performance.
AAMI car policies automatically include cover for modifications and accessories up to the amount your vehicle's covered for. There's no need to list them on your policy, just make sure the amount you cover your car for is enough to also cover the value of all the insured accessories and modifications.