Where should (or it is recommended that) is used, this indicates that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required.
Normative elements set out the provisions to which it is necessary to conform in order to be able to claim compliance with the standard. The text in a normative element usually contains both shall and should.
In IEC 61508, the following contain normative elements: part 1 (excluding annexes); part 2 (including annexes); part 3 (including annexes A and B, excluding annex C); and part 4 (excluding the annex). There are no normative requirements in parts 5, 6 and 7 of the standard.
Informative elements of the standard provide additional information intended to assist its understanding or use, but with which it is not necessary to conform in order to be able to claim compliance. The text in an informative element cannot contain shall. Notes and footnotes are always informative.
In IEC 61508, the following are informative: the annexes of part 1; annex C of part 3; the annex of part 4; and all annexes of parts 5, 6 and 7.
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