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. gnome-dash is still an unimportant tag that could easily be removed. The only reason that gnome-dash exists at all is that is so that Ubuntu 17.10 and 18.04 users aren't forced to use the unity-dash tag.