The Assembly Concept is a system of categorizing work based on the major components of a building project. This is a useful way of thinking about your project for many purposes - storing typical details, organizing design work, estimating costs, structuring bid packages, planning construction.
The old 16 division (now many more) CSI system with its many sections represents the final work breakdown by individual components. This is helpful when you are looking for completeness or trying to describe, price or specify those components, but in many ways and for many tasks this fine detail is an impediment. For example, a window section in a masonry wall involves Divs 4, 5, 7, 8, 9 for masonry, lintel, flashing, window, and finishes respectively. It is very difficult to think about the window detail in terms of those five divisions, but the detail fits very nicely into one of the Assembly Divisions in this system - Exterior Closure.
Conceptually, with Assembly Divisions, each part of the work or building project falls within just one of the divisions. This facilitates thinking about or sorting project issues.
When using Assembly Divisions, a project is divided into the following 12 categories.