Buttons in the new Email Design Center allow for creating links that are visually interesting, and distinct from linked images or text in an email. Because they are visually distinct, buttons are great as a call to action - "RSVP" or "Click here to register".
To create a button, select Button Editor from the toolbar.
Click to edit the text of the button. Buttons have the [INVITE_LINK] Dynamic Element - for Invite to RSVP, Invite to Register, and Invite to Purchase Ticket Types - linked by default, but can be replaced with any URL.
Buttons can have borders, with colors, widths, and styles. Border Rounding makes the button corners more square or more round, and Height Padding and Width Padding make the button taller and wider. The text color, and the button color, are also adjustable.
To edit a button already in the email design, select the button > Button Editor. To change the URL for the button, select the button > Edit Link.