!!! Overview[1]
In this scenario a user attempts to access a protected resource directly on an [SP] Web site without being logged on. 

The user does not have an account on the [SP] site, but does have a federated account managed by a third-party [Identity Provider (IDP)]. The [SP] sends an authentication request to the [Identity Provider (IDP)]. Both the request and the returned [SAML Assertion] are sent through the user's browser via HTTP POST.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [SP-Initiated SSO-POST-POST|https://documentation.pingidentity.com/display/PF610/SP-Initiated+SSO--POST-POS|target='_blank'] - based on data observed:2015-05-18