TLS certificate iCloud Mail.app

On one of my Macs, I have the option to select a TLS certificate in my iCloud account in the mail.app.

This option is under the SMTP server settings. It doesn't show up on my other Macs though. It is set as none in the aforementioned computer but I have the option to select a certificate.

Should I select it or not? Assuming it was set as none, does that mean that my information wasn't encrypted between my computer and iCloud?

Thanks.