Hello
Thank you for your question and reaching out.
Only if they use a service other than IIS will you need CAL. If your authentication is linked to AD, or, for example, if they are using the server DNS or DHCP. I don't think you'll require them.
Instead of hosting a site specifically for a business partner, an external connector licence is used when you want to give them access to your network or website. You will require the Services Provider licence if you are hosting a website for them as opposed to granting users access to your website. This is how I interpret the distinction. I hope this helps you understand it at least a little bit more.
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