What is the App Drawer's "Sort Applications = Index" Option about?

[just-Nob]

I don't know what is the apps' index about that can be invoked by the three-dots-menu and chosen in the app drawer as an alternative sorting method by this dialogue:

Can anybody explain, please?

And why there's the "Rearrange Applications" switch available for this, which isn't available for alphabetical order? - Is the alphabetical order rearranged automatically if switched from "indexed", whereas the indexed order must be forced to be rearranged?

[JackEblan]

Index is used by sorting items. If you turn on the Rearrange Applications you will see a new screen where you can drag and drop the applications to rearrange it. Don't forget to click the Floating Action Button at the lower right corner and Save your arrangement.