In a non-federated suite of applications where the applications and authentication mechanism is controlled by a single institution,  logout is a simple requirement to implement. 

In a federated SSO scenario, to implement logout functionality, the following must be answered:

• At which college/district IDP did the user authenticate?
• If a user is logged into to multiple applications, does logging out of one application mean the user should be logged out of all applications?  For example, if the user is logged into MyPath and Canvas, does logging out of MyPath also mean the user should be logged out of Canvas?

Reference

http://xacmlinfo.org/2013/06/28/how-saml2-single-logout-works/