It's in the third paragraph of the Wiki article you referenced: "Many Chicago School skyscrapers contain the three parts of a classical column. The first floor functions as the base, the middle stories, usually with little ornamental detail, act as the shaft of the column, and the last floor or so represent the capital, with more ornamental detail and capped with a cornice."
I was aware of the rest of the wikipedia entry - so you'll have to accept that I am not depending on that entry as the last word on tripartite buildings as an identifier of a Chicago School building.
For anyone interested, there is a larger and more complex explanation for the Chicago School - it begins with Carl Condit in the 1960s, objected to by H. Allen Brooks, and has a long list of twists-and-turns that carry through to today. While it is true that Louis Sullivan's "The Tall Office Building Artistically Considered," written in 1896, is often cited as the source for the assertion that tripartite design is a signpost for the first Chicago School, what it fits best is his work more than it fits the width and breath of the first Chicago School.
The counter-explanation, as far as I can reduce it, is that the Chicago School was not monolithic, but rather the result of a convergence of technology (perfection of the lifts, steel framing etc), opportunity (the Great Chicago Fire destroying older buildings in the central business district, eventually jump starting a boom in new skyscrapers) and the demands of a no-nonsense business clientele (build well but build efficiently). Some adopted the tripartite design, some experimented with less stylized bases, tops, and ornamentation, in varying degrees, all taking place in a charged environment that unfolded dramatically there on the shores of Lake Michigan. This research reaches below the stars of that day - Sullivan, Root, Atwater etc, - and looks into a larger pool of extant skyscraper proposals to see if there is something beyond theory in the practice of the craft. Was the Chicago School a style or something broader? Something broader is what I get out of these studies.
If I were forced to compare
Chicago in Mississauga, with some building from that era, it would be
The Manhattan Building, still in Chicago's South Loop.
The Manhattan Building is significant for what you cannot see: it was one of the first tall buildings to use steel frame construction. But on the design front, the
Manhattan was and still is a mess. It happens to be a tripartite building that had poor detailing and uninspired transitions between segments - a textbook lesson in what not to do. If it were anywhere else in the city, rather that the historic tall-building district, it might not have survived past a hundred years as it has thus far - technical distinction could only carry it so far.
In my opinion,
Chicago in Mississauga
repeated that lesson in what not to do with a tripartite design, and it will pay a bitter dividend for it.