A ConcurrentNavigableMap has all the methods of a NavigableMap, plus all the methods of a ConcurrentMap. Other NavigableMaps do not have ConcurrentMap methods.
Is it ConcurrentNavigableMap can be modified by different threads only?
No - if for some reason you want to modify a ConcurrentNavigableMap using only one thread, that's fine, it will work normally. The point is that if you do have multiple threads accessing a Map, you need a ConcurrentMap.
subject: difference between NavigableMap and ConcurrentNavigableMap