PowerBOM assists you in preparing the bill of material by disallowing duplicate numbering.
When you assign an item number that already exists in the document, following box is displayed:
If you press OK, all assigned numbers are shifted to the next one.
Further you can press Shift Up button in the bill of materials tab to shift all numbers by (-1). This is required, for example, when you have deleted a drawing item and you don’t want to leave the number blank.