I work in the IT industry and often read software and standards specifications that start with a section with definitions for certain words used in the document.
Recently I came across the following in section 1.3 of pdf document SSEK Version 2.0 by Andersson et al [verbatim except for added dashes]:
The keys words SHOULD, MAY, MUST and MUST NOT in this document are to be interpreted as follows:
SHOULD – This word mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
MAY – This word mean that an item is truly optional.
MUST – This word mean that the definition is an absolute requirement of the specification.
MUST NOT – This phrase mean that the definition is an absolute prohibition of the specification.
The question is whether the last definition really is correct, or if it should have been MAY NOT instead to have the defined meaning.