Warranty
When arranging liability insurance, along with other financial estimates, you will need to provide your insurer with an estimate of annual payments to “Bona fide” sub-contractors. Bona fide sub-contractors are generally deemed to be contractors who work without your direction. They hold their own insurances and will usually provide their own materials and tools.
An example might be a specialist Demolition Contractor brought in to demolish a building as part of a larger contract. Although bona fide sub-contractors might carry their own insurances, their activities could lead to a claim being lodged against you and you will need to insure against this "contingent liability".
For example, they could do something outside the scope of their insurance cover that leads to a claim or they may have simply not paid their premium.
Additionally, it could just be that a third party decides to make a claim against you for the negligent actions of one of your bona fide sub-contractors (rather than against them directly), which you will need your insurer to defend.