The chances of winning the case in question must be better than half. For civil cases, the prospect of recovering losses or receiving damages must be more than 50% and for defence cases the chance of being successful must be 51% or more. This cover excludes criminal costs e.g. private prosecution. A full list of exclusions can be found in the policy wording.
The chance of success will be decided by the insurer or their preferred 3rd party on their behalf.