Application is almost same as D ring as below.

In industrial manufacturing:
D rings are commonly found in machinery, providing connection points for belts, chains, slings, and other fasteners to ensure safe and efficient operation;

In the aerospace industry:
D rings are used to secure and connect aircraft interiors, seat belts and safety equipment, as well as components in cargo plane cargo holds;

In military and defense applications:
D rings are also commonly used to secure equipment such as backpacks, parachutes, tactical vests, and other military gear, for securing seat belts, harnesses, and so on;

In the civilian sector:
D rings are widely used in backpacks, pet leashes, hammocks, and more.