The ISM Code definition is “an observed situation where objective evidence indicates the non-fulfilment of a specified requirement” – in other words, any departure from an instruction or procedure documented in the SMS.
For example, if the SMS requires that before commencing bunkering operations, communications with the bunker barge are agreed and tested, but this is not done, there is a non-conformity. If the SMS requires that a checklist be completed, or a weekly drill or test be carried out, but this is not done, there is a non-conformity.
