A stackable is a type of item which may be collected into a group in the booty panel. Unlike other items, for which one icon in the inventory represents a single item, a stackable item may have several items represented by one icon - the number of items represented is indicated by a small number in the upper left corner.
Stackables may be traded like any other item, however to trade only a single item instead of the entire group, the group must be split apart. To do this, simply click on the icon representing the group, and select "split item group" from the item menu.
A dialogue box will pop up asking how many items should be split off. If a user attempts to enter a number equal to or greater than the current size of the group into the dialogue box, it will automatically change to be one less than the size of the group.
Clicking "ok" will create a new group for the same item with the chosen number of items in it. Any number of groups may be created like this, save that a group with a single item cannot be split. To re-merge the groups, simply select "combine into group" and all groups of that particular item will be merged into one group.