Security Assertion Markup Language (SAML) holds the dominant position in terms of industry acceptance for federated identity deployments. SAML is deployed in tens of thousands of cloud single sign-on (SSO) connections. Thousands of large enterprises, government agencies and service providers have selected it as their standard protocol for communicating identities across the internet.
SAML is XML-based which makes it a very flexible standard. Two federation partners can can choose to share whatever identity attributes they want in a SAML assertion (message) payload as long as those attributes can be represented in XML. This flexibility led to pieces of the SAML standard, such as the SAML assertion format, being incorporated into other standards such as WS-Federation.