Which term refers to a line that defines surface areas for coordinating operations between adjacent units?

The term that refers to a line defining surface areas for coordinating operations between adjacent units is a boundary. Boundaries serve as essential demarcation lines that delineate the operational areas of different units. They ensure that each unit understands its area of responsibility and can manage resources and strategies without overlap or confusion.

In the context of military operations, having clearly established boundaries helps to prevent fratricide and enhances coordination among units. Each unit is aware of the limits of its operational space and can plan effectively within those parameters, thereby supporting overall mission success. This is crucial in complex operational environments where multiple units may be engaged.