There’s an unwritten rule that says when describing a network architecture the perimeter of the data center is at the top. Similarly application data flow begins at the UI (presentation) layer and extends downward, toward the data tier. This directional flow has led to the use of the terms «northbound» and «southbound» to describe API responsibility within SDN (Software Defined Network) architectures and is likely to continue to expand to encompass in general the increasingly-API driven data center models.
But while network aficionados may use these terms with alacrity, they are not always well described or described in a way that a broad spectrum of IT professionals will immediately understand.