The key words “must”, “must not”, “required”, “should”, “should not”, “recommended”, “may”, and “optional” in this report are to be interpreted as described in RFC 2119 . Specifically:
In particular, this report occasionally uses “should” to designate circumstances that are outside the specification of this report, but cannot be practically detected by an implementation; see section 5.4. In such circumstances, a particular implementation may allow the programmer to ignore the recommendation of the report; it may even exhibit reasonable behavior. However, as the report does not specify the behavior, these programs may be unportable, that is, their execution might produce different results on different implementations.
Moreover, this report occasionally uses “required” to designate circumstances that are an absolute requirement of the specification, equivalent to “must”, and “not required” to note the absence of an absolute requirement.