What does a mullion strip do in combined window configurations?

A mullion strip serves to separate units in combined window configurations. When multiple window units are designed to be installed together, the mullion acts as a vertical or horizontal support that delineates the individual windows, thereby maintaining structural integrity and aesthetic coherence. This separation is essential for both functional and design purposes, as it ensures that each unit operates independently while remaining cohesive as a whole. By providing this division, the mullion also contributes to the overall stability of the window structure and helps diminish the risk of water infiltration between the units. The other options, while related to window performance and functionality, do not accurately describe the primary role of a mullion in this context.