Contingent fee arrangements for non-assurance services

Dear tutor, I learned from the text book that contingent fee arrangement is not allowed for audit and assurance services. But I could not find anywhere that this arrangement is either prohibitted for non-assurance services. However, in the KIT it' said: "The ACCA Code of Ethics and Conduct states that contingent fee arrangements are not allowed for non?assurance services provided to audit clients if the fee will be material (significant) to the firm, therefore option 4 is the most appropriate answer." Can you please explain the case of contingent fee arrangement for non-assurance services? Thank you in advance!