Roll up Tags are created on Plan Boards. These are high-level summary tasks.
You can create them by clicking on the 'Create tag' in the Tag Shelf at the bottom of the board.
Enter the name, description for the tag and then check the 'Rollup Task' checkbox
Clicking on the 'Add' button will create the tag as a Rollup task.
When you are creating a tag on a dispatch board, these Rollup Tasks show up as a list in the 'Scheduling Rollup Items' field. All the tags created by selecting a roll-up task in the 'Scheduling Rollup Items' get listed under the same Rollup task when the board is grouped by 'Rollups'.
Last Updated: Dec-09-20