Whole-page CMS is the easy case; real apps want ONE editable band inside an otherwise hand-coded screen — a promo strip, a help panel, a hero. Re-rendering the full document for that (or iframing a page) is the wrong altitude.

CmsRegion renders a single subtree of a PageDoc: pass root (any node id) and it renders that node plus its descendants with the same element registry and injected styles as CmsPage. Stable node ids make every region individually addressable — by your code and by agents.

Keep one doc for a surface group, then drop <CmsRegion doc={doc} root="promo-band" /> wherever the region belongs. Same registry / includeStyles knobs as CmsPage; an edit made in the Editor shows up everywhere the subtree is mounted.