The verbal form „soll“/„soll nicht“ (“shall”/”shall not”) is used to indicate requirements strictly to be followed and from which no deviation is permitted. German standards (e.g. DIN) prefer „muss“/„darf nicht“, whereas English standards do not use “must” as an alternative for “shall” in order to avoid any confusion between the requirements of a standard and external statutory obligations. Other equivalent expressions are „ist zu”, „ist erforderlich”, „es ist erforderlich, dass”, „lediglich … ist zulässig”, „es ist notwendig” / „ist nicht zulässig [erlaubt][gestattet]”, „ist unzulässig”, „ist nicht zu“, „es hat nicht zu“.
The verbal form „sollte“/„sollte nicht“ (“should”/”should not”) is used to indicate 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, or that (in the negative form) a certain possibility or course of action is deprecated but not prohibited. Other equivalent expressions for use in exceptional cases are „es wird empfohlen, dass …”, „ist in der Regel …” / „wird nicht empfohlen”, „sollte vermieden werden”.
A document does not in itself impose any obligation upon anyone to follow it. However, such an obligation may be imposed, for example, by legislation or by a contract. In order to be able to claim compliance with a document, the user needs to be able to identify the requirements he/she is obliged to satisfy. The user also needs to be able to distinguish these requirements from other provisions where there is a certain freedom of choice.
Clear rules for the use of verbal forms (including modal auxiliaries) are therefore
Source: ISO/IEC Directives — Part 2: Rules for the structure and drafting of International Standards (2011-04)