Occasionally your Herschel stock figures might be found to be incorrect. 

The adjust stock option allows the stock at each location to be increased or reduced. Each adjustment should ideally be authorised by some kind of document (sometimes called an adjustment authorisation):

When stock levels are found to be inaccurate you should always try to determine how the discrepancy arose. Has a genuine error been made?  Are your internal procedures watertight? Accurate stock levels are a must, not a luxury, especially if you intend using the Plan module.