In a set of mutually exclusive tags, the first tag can be defined as a group tag. When you search for a group tag, it will return matches for all members in the group. In an agenda view, filtering by a group tag will display headlines tagged with at least one of the members of the group. This makes tag searches and filters even more flexible.
In this example, ‘@read’ is a group tag for a set of three tags: ‘@read’, ‘@read_book’ and ‘@read_ebook’.
You can also use the :grouptags keyword directly when setting org-tag-alist, see the documentation of that variable.
If you want to ignore group tags temporarily, toggle group tags support with org-toggle-tags-groups, bound to C-c C-x q. If you want to disable tag groups completely, set org-group-tags to nil.