For POS transactions, this field in the Open Item Maintenance window is the cost of goods sold based on the cost in Inventory Maintenance.
For transactions you enter in the Open Item Maintenance window, this is the cost that you enter or the cost that the system calculates.
When adding transactions, you can enter a cost. If you don't enter a cost, the system calculates it by using the Gross Total and GP % fields. (The GP % comes from the option Minimum Gross Profit % [ID# 103] in Options Configuration.)