Supplements in liability policies are best described as:

Supplements in liability policies are indeed best described as provided in addition to other coverage limits. This is an important feature of liability insurance designed to enhance the policyholder's protection. Supplements can offer extra monetary limits for specific types of claims, such as defense costs or specific types of damages, which are not included in the basic limits of the policy.

By offering additional protection, these supplements help ensure that policyholders have adequate resources to respond to liabilities that exceed their primary coverage limits. This is particularly crucial in situations where legal defense costs or other related expenses can quickly escalate, allowing policyholders to maintain financial stability even in challenging circumstances.

The other options do not accurately depict the nature of supplements in liability policies. For example, describing them as part of the amount of insurance fails to recognize that they serve as additional layers of coverage rather than being included in the initial policy limits. Similarly, suggesting that they only insure costs of defense overlooks their broader applicability in providing extra coverage, while saying they cover the cost of emergency medical aid inaccurately narrows their function to a specific type of expense.