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Header (also known as separator) allows to visually split checklist into parts or groups as presented on the screenshot below:
When adding a new item or editing an existing item there is an option to save text as a separator with the button:
When adding a new item, editing an existing item, using Editor or Checklist Text custom field, there is an option to add a separator by preceding the text with three dashes "---":
You can also use the multiple checklists feature to separate checklists even more.