There are many strategies for laying out a content area with top/bottom or left/right panels, but most of these assume that the side panels have a known, fixed height or width (respectively). The SimpleFlexBox control works with panels whose content size is not determined until runtime.
This control is a "polyfill", meant to enable use of layout features which are not yet available in all browsers. For simplicity, the control assumes a maximum of one panel to either side of the main content panel. I.e., in horizontal orientation, there can be up to one left panel and one right panel; in vertical orientation, there can be up to one top panel and one bottom panel.
This control was discussed in a user interface blog post.