What defines the unusable portion of the runway?

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The unusable portion of the runway is primarily defined by runway edge markings. These markings serve to indicate the boundaries of the runway and highlight areas that are not intended for aircraft operations, such as areas that may be unsafe due to obstructions, maintenance, or uneven surfaces. By clearly demarcating the edges, these markings help pilots maintain proper alignment and ensure safety during takeoff and landing.

The other options focus on markings that serve specific purposes related to different phases of flight or aircraft positioning but do not delineate the actual boundaries where aircraft should avoid operating. For example, taxiway markings are crucial for navigating from the runway to other areas of the airport, while aiming point and touchdown zone markings help pilots with landing and stopping distances but do not define unusable areas. Thus, runway edge markings are essential for informing pilots about the limits of operational runway space.