The one genuine gap in modern information architecture. Nesting means one site OWNS another; association means independent sibling sites each declare that they belong to hub H. This platform has the first completely and none of the second, and HubPortal is neither - it unions list names platform-wide with no membership concept, so it cannot be scoped to just these sites. Use SharePoint's own name, HubSiteId, matching Register-SPOHubSite and Add-SPOHubSiteAssociation. THE ASK-FIRST ITEM IS NOT THE ATTRIBUTE, IT IS THE READER: the site manifest has no XSD, so the attribute would validate and do nothing, which is the silent-no-op class the fidelity survey ranked most expensive. Do not declare it without the consumer that filters the rollup by membership.