Construction Industry Terminology (David Standen, 2000 page 320) defines Professional indemnity insurance as:

Insurance taken out by a professional practice against the risk of paying for claims arising from breach of professional duty by reason of negligence or other specified causes. The insurance in any particular case depends on the policy and is usually wide enough to embrace (1) acts of employees as well as principals where the employees act in the course of their employment, (2) duty in contract, to clients, and (3) duty in tort, to others. Cover is provided for the ususal activities of particular classes of practices.

Cover in respect of unusual activities of one practice or a small proportion of practices within a class is difficult to obtain and in some cases, not available. For example an architect's practice may obtain cover in the case of activities that are ordinarily performed by architects but if the practice engages in, say, project management or building it may find that its cover does not extend to such activities or that extra cover is available only at a very high cost. This is because the departure from normal activities of the same class of practices increases the difficulty of the insurer in assessing risk.