How are materials assigned in OpenStudio?

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I if understand the question correctly, you are asking how to assign Constructions to Surface Types and (maybe) how to build a Construction from various Materials.

There are two ways to assign a surface: one is to go surface-by-surface, giving each an individual construction. The other is in the Construction Sets tab, to assign a particular Construction to each Surface Type (window, exterior wall, interior wall, etc.).

The Constructions themselves have their own tab, on this tab, each Construction is defined as a build-up of various layers of materials.

On the Materials tab, individual material properties can be adjusted. If you edit a Material, that update is reflected in any Constructions that the Material is used in, which, in turn, updates the Construction Set containing any of these Constructions.