Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT, "RECOMMEND", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119] and [RFC8174] when, and only when, they appear in all capitals, as shown here.

The DITA specification uses <keyword> elements with the @outputclass attribute set to RFC-2119 for these key words. In general, normative statements that use such key words pertain to what is needed for interoperability.