In discussion with @maryjom about WCAG2ICT work, we noted that Reflow lacks both a preamble giving it context, as well as clearer guidance on how to apply at different form factors.

Setting aside the latter concern for now, I believe that a simple preamble could be added that which would provide guidance for non-web application generally, and potentially for some web applications that are locked down.

The approach is to add a simple preamble, "Where the size of the viewport or the size of the content can be altered...", to the existing text, so that the SC reads:

Where the size of the viewport or the size of the content can be altered, content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions..."