An indexed collection has a collection of indexes, ScannerCollection<T> that are maintained up-to-date on every change made to the collection.
This interface is implemented by all main LiveLinq collection classes: C1.LiveLinq.Collections.IndexedCollection<T>, C1.LiveLinq.AdoNet.IndexedDataTable<TRow>, C1.LiveLinq.LiveViews.View<T>.
You need to implement this interface only if you want to define your own indexable collection classes and then only if they don't inherit from C1.LiveLinq.Collections.IndexedCollection<T>, see LiveLinq to Objects: IndexedCollection(T) and other collection classes.