In some projects/markets these are mandatory deliverables (the designation and the acronym can vary): SIS (Software Interface Specification), ICS (Interface Control Specification), ICD (Interface Control Document). The interface specification can be a document by itself or part of the broader design documentation (a section of it). The advantage of having it extracted is that it is easier to share externally (you'll have to share just that part of the design with external companies and that eases up distribution issues for organizations that restrict distribution).
As per the external interfaces by themselves:
Identify all intervening external systems and specify the needed interfaces thoroughly: specify the data flows, frequency and means of communication, review and accept those documents so that all teams (persons) working for all components of the information system are aligned.
Case study / example (on the importance of interface specifications):
The Mars Climate Orbiter is one of the many failed Mars missions. It crash landed in Mars in 1999:
"The primary cause of this discrepancy was that one piece of ground software supplied by Lockheed Martin produced results in a United States customary unit, contrary to its Software Interface Specification (SIS), while a second system, supplied by NASA, expected those results to be in metric units, in accordance with the SIS."They had a spec, they just didn't read it. Or even worse, they produced contradicting documentation.
PS. The full NASA 1998 report can be found here:
