A template holds what does not change from one send to the next: header, logo, footer, address, unsubscribe link, styles. The campaigns and automation e-mails that use it only fill its editable zone.
Create a template
Templates → New template, choose an editor (blocks, Markdown, HTML or MJML), then give it a name.
The editable zone is written:
In the block editor, it is the Editable zone block. While the template has none, a warning says so: without a zone, the campaigns’ content appears nowhere.
An example in HTML
Put the unsubscribe link and the postal address in the template: every
campaign that uses it is then in order, with nobody having to remember.
style attributes, and the media queries are kept in the header.
Use a template
In the content editor of a campaign or an automation e-mail, choose it under Template. The preview shows the content inside the layout at once.
Change or delete
Changing a template changes how the next sends of every campaign that uses it look. Campaigns already sent keep their content: deleting a template does not touch them.
The template list shows the editor, the editable zones it found and the date it was last changed. 
