It stands for against the box; a short sale which is carried out by the holder of a long position in the same stock being sold. The stock is said to be sold against the box when it is sold short. Usually, the short seller doesn’t own the shares. However, in an against the box selling, the seller does own the shares. Investors unwilling to disclose ownership would prefer to follow this tactic or perhaps when the long stocks are inaccessible for delivery in the period of time required by the buyer.