This request header contains the mode of the layer targeted for a failed fragment update.
A fragment update is considered failed if the server responds with a status code other than 2xx, but still renders HTML.
Server-side code is free to render different HTML for different modes. For example, you might prefer to not render a site navigation for overlays.
The user may choose to not send this header by configuring
X-Up-Mode: drawer X-Up-Fail-Mode: root