On the email template configuration page, there is a section for configuring Application Settings. You would configure these under the Settings tab in "Autofill Settings" and "Attachment Settings".

When you are on an Event Information Page for a given event, there is a drop-down where you can select from the available Document and Email templates. When you select an email template, you are taken to the compose email page. On that page, there is a drop-down where you can attach a document to the email. If you have selected an attachment autofill document on the email template settings page, this document will automatically be selected.

Here is one situation where this would be helpful. Say you send a timeline to your clients for their approval. You would create an email with the wording that asks them to review the attached contract and get back to you if they need any changes. Whenever you send this email, you would previously attach the timeline manually by selecting it from the attachment dropdown on the compose email page. With the new Attachment Autofill setting, that document will automatically be selected in the attachment dropdown.