There's two things to consider
The Mendix Portal now offers the option to bring your own IdP. This is what your screenshot refers to. This will depend on the company you belong to.
Mendix Apps can be extended with different SSO mechanisms, including the MendixSSO module, which allows to delegate authentication in your App to the Mendix Portal.
Combining both allows to login to an App with MendixSSO through your company IdP registered in the Mendix Portal.
SSO is enabled for the accounts associated with your company.
It's not bound to specific applications, so if you invite externals to a project they will still use their own login mechanims.