SDC: StructureDefinition-sdc-rendering-criticalExtension-intro

If this element appears at the root of a resource, then the expectation applies to all uses of the extension/element throughout the resource. If it appears on another element, the expectation for support of the extension/element only applies to the element on which the rendering-criticalExtension extension appears, and any descendants of that element.

If this extension is present, the instance SHALL populate the Resource.implicitRules element with a hyperlink pointing to this extension definition, with an expectation that systems will adhere to the expectations of this extension when presenting the resource.