All will yield steel, albeit of varying quality, quantity, and cost. Thus among the unvarying structural truths about steel is that regardless of what process is used to make it, steel is made from some proportions of several raw materials. Our interest in steel may not include the raw materials or proportions but their existence is undeniable and universal. A complete data structure designed to describe steel must make provision for recording the materials used. In another example familiar to many readers, people also have data properties whose values vary but whose structure is universal.
All people have gender, height, weight, eye color, etc.
And for many years bureaucracies have also deemed that people are single or married. Recently, however, many institutions have been forced to recognize that marital status reflects social processes, rather than human physiology. A person may be unmarried yet in a permanent relationship with another of the same or different sex. Not only does our society struggle to adjust to unconventional forms of personal relationships but in a smaller way information systems are stretched and twisted to record unanticipated facts which were, for example, mistakenly attached to people when those facts belonged instead to relationships. This human example demonstrates a common flaw of information systems which fail to differentiate a process-defined view from the underlying structure of things.