Roll up Tags are created on Plan Boards. These are high level summary tasks. You can create them by 

checking the 'Rollup Task' checkbox and then click on the 'Add' button. 

This tag along with other tags created in similar fashion get listed under the selected 'Scheduling Rollup Items' field, in the 'Add Tag'  page of a Dispatch Board.

These tags are listed under two sections,

Last Updated: Dec-09-20