The Item Number box displays the internal "Item Number" of each item.  Item numbers are assigned automatically by the system and cannot be changed.  


The system uses the item number to assign file names to pictures when they're imported.  For instance, if you add a picture to Item Number "1," picture files 1.jpg and 1l.jpg will be created in your images directory.  That's how Inventory Express knows what picture goes with what item.


If you had sorted your inventory by clicking on a Grid Heading, clicking on the Item Number box will return your inventory to it's natural order (the order in which the items were originally entered.)