The FIN 5.1.1 release is considered a 'service pack' release. Service Packages typically contain minor bug fixes and feature enhancements
Notable Fixes & Enhancements
OpenID Connect SSO
Single sign-on using an external identity provider: Single sign-on (SSO) can be set up using OpenID Connect. This enables users to skip the standard FIN username and password and use an external identity provider (IdP).
Single sign-on using FIN: If multiple FIN instances are being used (and Edge2Cloud isn’t being used), a single FIN instance can manage users and provide a single sign-on (SSO) to all other instances of FIN. This uses OpenID Connect technology whereby FIN can act as both a server and a client.