A fault claim is a claim where your own insurer has had to either pay out the full cost or a proportion of the cost for a claim made themselves.
This means that that it can be any claim where 100% of the costs paid out cannot be obtained from a third party’s insurance company, which can be for several reasons such as the insurers not being able to prove who was fully responsible for an incident for example.
It would also go down a fault claim for example if there is only a single vehicle involved in an incident or also in the case of a theft, which due to there usually being no other party to claim off will both result in your own insurer having to pay out for the costs themselves.