In https://www.hl7.org/fhir/profiling.html#mustsupport  document that... If a child is MS and the parent isn't, then if you support the parent you must support the child, but there's no expectation that you must support the parent. If an IG defines a complex element as MS, and does not declare any of its children as MS, then the IG should provide further guidance; if there is no further guidance, then the expectation is that implementers must support at least a subset of the child elements of the MS parent.