Every plan lists its exclusions in the policy wording. Typical examples are cosmetic surgery, pregnancy and childbirth unless the plan has a maternity benefit, self-inflicted injury, and treatment that is not medically necessary.
Some exclusions apply only in certain cases, such as pre-existing conditions in small groups or hazardous activities under personal accident cover.
Exclusions differ between insurers more than most people expect, so they belong in any comparison alongside the limits and the price.