question about index in Hana

As far as i know that hana can automatically create a "reverted index" in column table. i also notice in SQL reference guide, that there also "create index" and also we can chage "revert index type (hash/value) " in unique constrains.

so my quesiton is:

1. what is the use case of sentence "create index", whether this index is hana's inverted index or classical DB's index

2. in which case, we should mannually create or build index to replace the one hana automatically create for us.

any hint is very appreciate