The dash tag is currently a synonym for the unity-dash tag, so all questions that are tagged with dash are automatically retagged with the unity-dash tag instead. Ubuntu 17.10 and later also has an applications dashboard which is also called the Dash. Although the new Dash looks the same as the old Dash, there is a new tag for it called gnome-dash instead of one tag for both.
This is not the expected behavior. It is awkward and cumbersome. A better way to do it would be to convert dash from a child synonym of unity-dash into its parent tag and make both unity-dash and gnome-dash synonyms of dash.
In case anyone disagrees with the existing names for screen real estate objects in Ubuntu 18.04, this is not what this question is about. If you want to ask about changing the names for screen real estate objects in Ubuntu 18.04 feel free to do so, but please ask about it in another question. This question is only about how we should tag the Dash, not about possible new names for the applications dashboard in Ubuntu 18.04. If you don't like the name Dash it's OK to suggest a new one, but that's the name it has for now and this question is about how to tag it, not what would be the best name for it.