In a book of this type it is impossible to cover all the sector guidance which, in any case, is expanding rapidly. However, the following are a few of the many documents which now proliferate. They are often referred to as “second tier” guidance in relation to IEC 61508. Due to the open-ended nature of the statements made, and to ambiguity of interpretation, it cannot be said that conformance with any one of them automatically infers compliance with IEC 61508. These documents tend to cover much the same ground as each other albeit using slightly different terms to describe documents and life-cycle activities.