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In DTP type applications, a rectangle with a pair of diagonal lines from corner to corner is used as a placeholder to signify blank space awaiting content, or not rendered.
Is there a name for this element?
It is simply known as a frame.
In InDesign you can create these with the Rectangular Frame Tool and in wireframing apps they are just called frames.
In spanish is "marco contenedor": frame container or container frame.
Required, but never shown