Editor Tabs Reference
Every kind of tab that can open in the centre workspace.
Why Units, Binder, Output and Rundown only open once.
These tabs edit page-level state rather than one object, so a second copy would be a second view of the same thing — and, for the Binder, a second place for an arming interaction to land. Asking for one that is already open focuses it instead.
That is also why the Poller and Map tabs disappeared: both were folded into Units, and old layouts are migrated to a single Units tab on load.