If the entry is an increase (a purchase or positive adjustment), this field shows the quantity that remains in inventory of the quantity in the Quantity field. If the entry is a decrease (a sale or negative adjustment), the field shows the quantity that remains to be applied to by an increase entry.
The program calculates the quantity automatically when the entry is posted.
When the entry is applied to, the field will be updated if you have used the Appl.-to Item Entry field on the sales line, the Applies-to Entry field on the item journal line, or the Adjust Cost - Item Entries batch job.