SharePoint in Microsoft 365 serves a wide range of customers with a variety of usability and security needs. Some customers don't mind asking users to reauthenticate if it means their data will be more secure. Other customers want to minimize the number of sign-in screens that users see, especially in situations where it seems as though SharePoint should already know who the user is. Luckily, customers don't have to choose usability or security because they work together in a lot of great ways.
The following diagram outlines the SharePoint authentication process. It walks through how the scenario works using either your own Identity Provider (IdP) or the default Azure Active Directory (Azure AD) IdP.
The Federation Authentication (FedAuth) cookie is for each top-level site in SharePoint such as the root site, OneDrive, and the admin center site. The root Federation Authentication (rtFA) cookie is used across all of SharePoint. When a user visits a new top-level site or another company's page, the rtFA cookie is used to authenticate them silently without a prompt. When a user signs out of SharePoint, the rtFA cookie is deleted.
Session and persistent cookies
By default, all SharePoint cookies are session cookies. These cookies are not saved to the browser's cookie cache and instead are deleted whenever the browser is closed. Azure AD provides a Keep Me Signed In button during login that passes a signal to Microsoft 365 to enable persistent cookies. These cookies are saved to the browser's cache and will persist even if the browser is closed or the computer is restarted.
Persistent cookies have a huge impact on the sign-in experience by reducing the number of authentication prompts users see. Persistent cookies are also required for some SharePoint features, such as Open with Explorer and Mapped Drives.
For more info about session timeouts, see Session timeouts for Microsoft 365.