Users in GLPI can and will use the simplified interface, as long as they are assigned the Self-Service profile. Each profile can be set to use either the Simplified or the Standard interface, thus users see an interface according to their assigned profile.

Simplified interface is the most restrictive, but also the easiest way to use GLPI and as such, there is a good general practice to not let users have any higher privileges than Self-Service. By default, users can create a ticket, see its treatment, react if needed and browse the FAQ sections. Also make reservations, however, not many folks seems to be using this feature.

Some scenarios of deployment allow managers and/or power-users to oversee tickets of their group. In that case, based on your user and group membership sync setup, carefully configure some other profile to be able to see group tickets and let those users use the Standard interface.