Official comment
Single opt-in: This means the contact only has to fill out the form and then they are opted in. You can start bulk emailing right away!
Double opt-in optional: This means that the contact only has to fill out the form and they are opted in as single opt-in. What is different though, is that it will also send a double opt-in email for them to click for confirmation. This doesn't stop you from bulk emailing your contacts, but it will allow you to see who is the more active user by who clicked the confirmation link.
Double opt-in required: This means that even though the contact filled out the form, before they can get any emails, they have to click the confirmation email to double opt-in. The contact will still be added to your ONTRAPORT system before confirming, but they will have a bulk email status of unconfirmed.
If you want to use double opt-in, you can access an ONTRAPORT Project step-by-step training on "Create a Confirmed (or Double) Opt-In Process to Build Your List." To do so, navigate to this link while logged into your account.