Expense Head in Purchase innvoice

Don’t really understand what expense head is related in purchasing?, Its mandatory to filled

If you are using perpetual inventory system it should be “Stock Received But Not Billed” which should come automatically if you defined it in company. In case of periodic inventory system, you should select “Cost of Goods Sold” account.

Ok Thanks, From the original ERPnext setup Chart of account (the stock received but not billed) is placed under liabilities. However, the expense head only shows the expense account. Should i move “stock received but not billed” into expense head in COA?