When "View Stock" is unchecked, this field is equal to zero.
When "Include Quantity in Inspection" is checked, the quantity on hand is equal to (the quantity on hand + the quantity in inspection).
If "Use Allocated Stock" is checked, this field is equal to (the quantity on hand - the allocated quantity).
If "Use Quarantine Stock" is checked, this field is equal to (quantity on hand - (allocated quantity + quarantine quantity).