CDO mode ensures that the data plane connectivity in the multisite environment. When primary site loses connectivity. Here you can enable CDO mode on secondary site to avoid any temporary connectivity issue related to data plane.
When the primary site is down or not reachable, the CDO logical switch is used only for control plane. Purpose and therefore its a not visible under logical switches tab.
CDO Mode in NSX Controller
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CDO ( Controller disconnect operation) Mode in NSX Controller.