What grooming requirement is specific for Marines' sideburns?

The grooming requirement that specifies Marines' sideburns is that they will not extend below the top of the orifice of the ear. This guideline is in place to maintain a clean and professional appearance that aligns with the standards of military conduct. By ensuring that sideburns are kept at this specific length, it promotes uniformity and contributes to a cohesive image within the ranks. This standard also allows for ease of wear with uniforms and headgear, ensuring that sideburns do not interfere with the proper fit of military headwear.

While other aspects of grooming, such as dyeing hair or connecting sideburns to mustaches, are also important, they serve different purposes and don't focus specifically on the length, which is the crux of this particular grooming requirement.