What does a double-insulated tool indicate about its construction?

A double-insulated tool indicates that it is engineered to provide a high level of protection to the user against electrical shock. This type of construction means that the tool has two layers of insulation separating the electrical components from the external casing. The first layer is the basic insulation that covers the electrical parts, while the second layer offers an additional barrier.

The absence of an external grounding feature, which is found in some tools that rely on a three-prong plug, means that double-insulated tools do not depend on grounding for safety. They are designed specifically to eliminate the need for a ground connection in normal operations, enhancing portability and usage in various environments without the requirement of a grounded outlet. The tool's casing might be made of plastic or other insulating materials, rather than metal, which further reduces the risk of shock.

The materials like rubber typically used in tool covers may contribute to overall safety, but they do not define the core concept of double insulation. Instead, the essential aspect of double insulation is its layered approach to insulation from electrical risks. Thus, the correct answer highlights this fundamental safety feature inherent in double-insulated tools.