Single Sign-On is a cloud-based single sign-on (SSO) service that makes it easy to centrally manage SSO access to all of your AWS accounts and cloud applications.
Specifically, it helps you manage SSO access and user permissions across all your AWS accounts in Organizations.
SSO also helps you manage access and permissions to commonly used third-party software as a service (SaaS) applications, SSO-integrated applications as well as custom applications that support Security Assertion Markup Language (SAML) 2.0.
SSO is integrated with Microsoft AD through the AWS Directory Service.
This is a much better option than AssumeRoleWithSAML because it can scale and it doesn't require you to set up an 3rd party identity provider (IdP).