Multi surface targets were added in https://gerrit.wikimedia.org/r/c/VisualEditor/VisualEditor/+/176952 as a way to support multiple VE instances on a page.

Neither Flow, nor its successor DiscussionTools ever used multi-surface targets (T121110 is dormant). Instead in various places we just load up multiple targets. As we have also come to need different types of VE target on the page, this is probably the better approach. (There are still some things to be fixed with this approach, namely global registries/singletons).

We still need to be able to have no surface assigned to a target (e.g. during loading), or to overwrite the existing surface (as used when switching from VE to source mode).