Available when the report is Grouped or Sorted By one of the available Group By / Sort By options (Department, Category, Vendor, etc.).
When Don't Print if Empty is enabled, and one of the Sort / Group categories doesn't contain any results, it is excluded from the report.
For example: If the Report is Grouped by Department, and there are no items in one of the Departments, that Department won't be included on the report.
Note: Also available on Sales & Purchases Transaction reports where if no line items are printed, the entire transaction will be skipped and excluded from the totals.
See:
Report Manager Contents