API stands for Application Programming Interface and the Email API allows an automated process to create and edit emails in Marketo. There are also other API calls that involve emails, such as Approve Snippet (assuming the Snippet is used in an Email) and Clone Program (assuming the Program contains Emails). There are also API calls to create and update Email Templates. Essentially, the API can do many things that you can also do through the Marketo user interface, but then in an automated fashion.
There are many scenarios: an external system could create Emails in Marketo using data that lives outside of Marketo. A translation service provider could clone a master Email, translate it to many languages, then save them back into Marketo as localized Emails. A reporting system could extract Emails from Marketo to use in reports that are generated outside of Marketo. An external system could Clone a Program that contains Emails, then populate the Program Tokens and schedule the Email to be sent out at a specific time. There could be an external email template creation system that creates new Email templates in Marketo through the API.
“Email Experience 2.0” is the new Marketo product feature with the enhanced email editor, documented here: https://docs.marketo.com/display/public/DOCS/Email+Editor+v2.0+Overview. It can be switched on in Admin > Email > Edit Email Editor Settings. All Emails and Email Templates also have a version number, either 1.0 (the old version) or 2.0 (the new version). If we refer to “Email 2.0 asset” we mean an email or email template in the new upgraded 2.0 format.
No. Enabling Email 2.0 will not automatically upgrade Emails or Email Templates to the new 2.0 format. The Email API can still create new Emails and Email Templates in the 1.0 format. However – after enabling Email 2.0 – any Email or Email Template that is created or edited and approved through the Marketo User Interface will automatically be upgraded to the 2.0 format.
If you edit an “Approved" or “Approved with Draft” 1.0 Email with Email 2.0 enabled, the draft is converted to the 2.0 format. You can still discard the draft to go back to the approved 1.0 format. Once you approve the email and it becomes 2.0, the Email cannot be converted back to 1.0. If you edit a “Draft” 1.0 Email (never been approved), this will automatically be converted to 2.0 with no option to revert back to the 1.0 format. The same applies to Email Templates.
If an Email or Email Template was accidentally converted to the 2.0 format, you’d have to copy the asset contents to a text editor, disable Email 2.0, then create a new 1.0 asset using the content that you copied.