In the Gantt diagram below, two tasks are in work interfaces. Their common point is the Zone, it is a Zone interface (geographical). it means that the two tasks are running in the same location.
The interface has a criticity (distance between 2 little 3D cubes), a complexity(function of the organization) and a duration (duration of the overlap).
All the tasks with overlaps are not in interface.
There are 3 natures of interfaces: Zone(geographical), Product (Physical) and Activity (Temporal).