Windows Authentication is quite popular but in real world, other authentication providers are needed. For example, some organizations require single sign-on (SSO) to provide end users with smooth experience across different platforms. Active Directory Federation Service (AD FS) meets the requirement, working as a shared trusted identity provider for the platforms involved. In SharePoint, it can be configured with Windows PowerShell after setting the SharePoint web applications as a relying party in AD FS. This sounds pretty straightforward. However, we could not get rid of Windows Authentications as crawling requires it.
It is possible to use two authentication providers in the same zone of the same web applications in SharePoint. However, this may not be ideal in the real world as users will be prompted the authentications provider picker before logging in if multiple authentication providers are enabled in the same zone. For example:
IT managers usually would like to avoid this as end users very likely do not know which one to choose and may get lost, which is not helpful for user adoption. In order to have users authenticated through an authentication provider directly without picking an authentications provider, there can be only one authentication provider in the same zone. Therefore, we need to extend the web applications into a second zone, and use the second authentication provider such as AD FS in that zone and Windows Authentication in the default zone.
You may have a question. Can we put Windows Authentication in the second zone and use AD FS in the default zone? The answer is no. if you crawl a zone of a web application other than the default zone, the query processor does not map search-result URLs so that they are relative to the AAM zone from which queries are performed. Instead, search-result URLS will be relative to the non-default zone that was crawled. Because of this, users might not readily be able to view or open search results. Therefore, when AD FS or any other authentication provider is required in a web application in SharePoint, the zone and authentications provider design would be something like in the table below.