The style of an index is created by a combination of features. Decisions have to be made about layout, punctuation, alphabetization, capitalisation, number of subheading levels, position and style of cross-references, how to give page ranges, and so on.
Publishers often develop their own guidelines so that the index can be created in their own house style.
Pinpoint Indexing is happy to prepare indexes in your preferred style. Or, if you wish, we can give advice on a particular style and layout for your index.
Some of the differences in index styles are illustrated here.