The abstract partition separating two or more items into individually identifiable things in the minds of the people working on, with, or around them. As a “logical device”, an I/F describes characteristics of the items, including (but not limited to) measures, states, physical characteristics, and functionality, any or all of which can be either coordinated or coincidental. The characteristics of interest here are those whose values are depended on by the party on the other side of the I/F.