The IG doesn't make it clear what "Must Support" means, but lacking that information, it seems to me that the IG uses the Must Support flag too liberally – ultimately increasing the burden of implementation. Each MS represents a non-negotiable requirement, and I have a difficult time believing that every one of these attributes must be supported for a functioning system (particularly some of the metadata fields; for example, must a system really understand the "codesystem-keyWord" extension to properly use a Code System in a Guideline definition? I doubt it – but tat is just one example of many.)