When you've selected multiple tasks that have different tags, the tags field will display <Multiple> -- you can erase this and type in new tags if you want to override the tasks' existing tags.
Alternatively, if you'd like to keep the tasks' existing tags, and just add new tags, you can type them after <Multiple> and the existing tags will be retained. For example, enter:
to add the tag 'supermarket' to all selected tasks, without losing the existing tags.
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