General Visibility
When you create or discover an API, the API is initially added to your organization. And is visible only to the members of your organization. You can add more organizations to this restricted view.
A public API is visible to all users of all organizations.
The general visibility also applies to the Developer Portal.
Developer Portal Visibility
With this setting you can publish the API in your Developer Portals. If you have multiple Developer Portals, this API is visible in the one that maps to the same organization.
If you have enabled guest access, the APIs that you want the guest to see must be public and published.