A special kind of structure combines multiple traits into a form that could be called a "functional trait group" (or maybe "functional forms" of traits). These traits are grouped together because they have a tendency to work out well when they co-occur, rather than because they correlate. For example, the brilliant 20th-century diagnostician Paul Meehl pointed out that certain distinct MMPI patterns occur much more frequently than others. Those patterns (he would argue, if I understand him correctly) occur together, again, because they function together to create a meaningful mental (or behavioral) pattern. Many people are more familiar with the typology of the Myers-Briggs Type Indicator. Certain MBTI profiles are likely to occur more often than others, and others are much rarer. For example, the INTJ type (an Introverted-Intuitive-Thinking-Judging type) is a fairly common type among personality psychologists, so I'm told, and would also potentially represent a functional form suited for such work, perhaps. (For example, introversion would promote studiousness.) The nomenclature "personality type" is unfortunate, to my mind, because it promises more than it delivers. What is really being referred to is a form or a set of functionally related traits selected by a theory (or measurement instrument) -- not the whole person. I make this point because I think many people would be more comfortable in discussing their MBTI type or other type if there were more common acknowledgement that this is not the whole personality we are speaking of.
Much work on the web concerning the Myers-Briggs Type Inventory (MBTI) concerns outgrowths of that Jungian typological thinking.
An example of such a site is Ross Reinhold's site on personality pathways, which focuses on the MBTI and related theories and concerns.
More recently, psychologists, urged on by Paul Meehl, have been developing new mathematical techniques to identify "typeness" and search for mathematical criteria of distinct types of individuals. As web sites on that research become available, I will add them here.