you can expand your inventory having vest... clothing or what ever in your inventory...
Example... your whole inventory is full of gear but put an vest in there (8-12 slot) usally double klicking doesnt do anything... but in 0.57 stable it places the double klicked item in the vest in your inventory. you can't drag anything in the vest but by double klicking the item get's placed in the free space in the additonal inventory...
not sure how I can it explaine better. xD {F31979}