Hi guys, i need your help with the redefinition of following function fields. I don't really get behind the store parameter in the 'amount' field. Any help or hints appreciated.

def _amount(self, cr, uid, ids, field_name, arg, context=None):
        res= {}
        for expense in self.browse(cr, uid, ids, context=context):
            total = 0.0
            for line in expense.line_ids:
                total += line.unit_amount * line.unit_quantity
            res[expense.id] = total
        return res


def _get_expense_from_line(self, cr, uid, ids, context=None):
        return [line.expense_id.id for line in self.pool.get('hr.expense.line').browse(cr, uid, ids, 
        context=context)]


'amount': fields.function(_amount, string='Total Amount', digits_compute=dp.get_precision('Account'), 
            store={
                'hr.expense.line': (_get_expense_from_line, ['unit_amount','unit_quantity'], 10)
            }),