A promotion is any transaction where a cost is incurred for the business but is not fully paid for by the customer. This is also sometimes referred to as a comp.
Example: giving a VIP guest a free bottle of wine is considered a promotion or giving a client 50% off all cocktails during happy hour.
All promotions are factored into gross sales and can be seen as % of gross sales, but are factored out of net sales.
A void does not incur a cost.
Example: a void could be an item that is ordered but then canceled before it is made and therefore incurs no loss.
A void is not factored into gross or net sales because it does not have any monetary value attached to it. Voids can be tracked to identify moments where time is being wasted.