A shared block keeps the parts every email has in common identical: your logo header, your footer, a signature. The rest of each template, such as the cover picture, the words and the button, stays that email's own content.
Open an email template, click the block you want to reuse, and press Save as shared block under its settings. Give it a name, such as Header, and press Save.
In any template, open the Shared blocks group at the bottom of the blocks list and click it, or drag it onto the email. A shared block has a dashed outline on the canvas.
Click the shared block and change it, on the canvas or in its settings. The panel says how many templates use it. Press Save: every template that uses it shows the new look in its next email.
Press Unlink on a shared block to turn it back into a normal block in this template only. The shared block and the other templates stay as they are.
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