This request will authenticate and redirect the user to the registered Assertion Consumer URL. Optionally, you can send in a RelayState parameter an 'acs' parameter as follows.
This request will authenticate and redirect the user to the URL in the RelayState 'acs' parameter itself.
Either you could have SP Initiated SSO only, or SP Initiated SSO and IdP Initiated SSO. You can't have IdP initiated SSO only. By design, SP Initiated SSO is more restrictive and secure. If a service provider is allowed to do IdP Initiated SSO, it would automatically imply that this service provider is allowed to do SP initiated SSO as well.