What does it mean when stock 'quantity on PO' is negative? I have no Open POs because they have all been converted to Bills. I'm used to seeing 'Qty on Hand' as negative if I have made a big sale and sold more stock than I currently have, and so I need to place a new stock order. But I have not seen 'Qty on PO' as negative before and don't know what this means (attached screenshot) do I actually have 28 items on hand or does this -18 on PO affect that and why? I need to know my stock levels accurately so I can place new stock orders.

Any help would be greatly appreciated? Thank you in advance