What is a significant feature of a grave liner in funeral services?

A grave liner is specifically designed to provide a protective structure for the casket buried within it, while allowing for proper drainage and air circulation. The significant feature of a grave liner is that it offers a degree of protection for the casket and the surrounding earth but does not seal it completely. This ventilation allows natural decomposition to occur and minimizes the risk of water pooling around the casket, which could cause deterioration.

While other choices mention sealing and memorial functions, the essence of a grave liner's purpose is its protective role without creating an airtight, sealed environment. This ensures that the grave remains stable and protects the casket from soil pressure, yet still permits the natural process of decay, aligning with environmental considerations in burial practices. Thus, it is categorized correctly as a method of offering protection without sealing.