Service for User (S4U) extensions allow a service to obtain a Kerberos service ticket on behalf of a user. Following are the two types of S4U extensions:
Service for user to self (S4U2Self). This extension allows a service to get a service ticket to itself on behalf of a client user.
Service for user to proxy (S4U2Proxy). This extension allows a service to obtain a service ticket to another service on behalf of a client user. To perform S4U2proxy, a service needs a service ticket to itself. The service ticket can be presented by the client user or obtained through S4U2Self extension.
For more information on the S4U extensions, see the Microsoft documentation.