This field in the Department Maintenance window is the cost of goods sold for items sold in this department. Each day, the Daily Inventory Report (RDI), Option F updates this field with the cost of goods sold of each item sold from that department.
The cost the system uses to update this field depends on option 410 "Use Current Cost When Invoicing Saved Transactions" in Options Configuration (or Function MIV).
If the field is Y, the system uses the Cost Used In Calculations field in Options Configuration (or Function MIC).
If the field is N, the system uses the cost entered in POS.