What is the purpose of creating index in Adobe frame Maker Document ?
An. Index is an alphabetic list of keywords with reference into the document (page number, hyperlink). See for example https://www.editorgroup.com/blog/what-is-an-index/ for the purpose of such a construct. FM can only provide the mechanics to create one - not the (hidden) intelligence required to create a good index.
For the mechanics you may look at https://www.daube.ch/docu/files/compendium.pdf pages 418 ff