What is the purpose of property semanticIndexCallback in SliverChildBuilderDelegate and SliverChildListDelegate?

Question

In the documentation, it is stated as:

A callback which produces a semantic index given a widget and the local index.

The statement from the documentation itself is hard to understand, so I am here for a help.

Answer 1

IndexedSemantics are used by assistive tools, more specifically by screen readers(TalkBack/Voiceover) to make announcements about the current scroll state.

Certain widgets like the ListView will automatically provide a child index for building semantics. As a developer you may wish to manually provide semantic indexes if not all child of the scrollable area contribute semantics. For instance, you can manually provide semantic indexes if you add dividers between the intended list items, as the dividers don't contribute semantics, you would want to avoid those items to be considered by the assistive tools.

https://api.flutter.dev/flutter/widgets/IndexedSemantics-class.html