Setting an email template as a default makes it so that email template will always load as the default when you're performing an action on the platform which aligns with that specific type.
You can only have one default email template per email type at a time, but do not need to specify a default.
There are two ways to set an email template as a default.
From your list of email templates, you can click the star icon to set an email template as your default.
After clicking the star icon, you'll be asked to confirm the change in a modal. Once you save the change, this email template will now be your default email template for that type.
Once you click Save, the email template will be your default for that type.
To remove an email template as your default, you'd simply edit the email template you want to change and uncheck the box next to Use as default before saving.