Individual websites or intranets may use a back-of-the-book index, while search engines usually use keywords and metadata to provide a more useful vocabulary for Internet or onsite searching.
[2] The implication with "A-Z" is that there is an alphabetical browse view or interface.
This interface differs from that of a browse through layers of hierarchical categories (also known as a taxonomy) which are not necessarily alphabetical, but are also found on some web sites.
Although an A-Z index could be used to index multiple sites, rather than the multiple pages of a single site, this is unusual.
This method is commonly used by search engine indexing.