The northbound interface conceptualizes the lower level details (e.g., data or functions) used by, or in, the component, allowing the component to interface with higher level layers.
A southbound interface decomposes concepts in the technical details, mostly specific to a single component of the architecture.
Southbound interfaces are drawn at the bottom of an architectural overview.
A northbound interface is typically an output-only interface (as opposed to one that accepts user input) found in carrier-grade network and telecommunications network elements.
For example, a device that is capable of sending out syslog messages but that is not configurable by the user is said to implement a northbound interface.