How can I find VS Web component slotting positions?

How might I find a reference for available slot positions in a Web component?  Might there be a registry in a component model somewhere?  I was playing around with ui.display-notification in a Workflow (which has no forms and targets "auto"... appears to be the whole app) and was wondering whether there's a reference somewhere.  Or maybe there's a registry in the components or their models that I could look at in the dev tools after binding to one?