The Item is a Key Flexfield which means that you can define it in the same way that you can define your Chart of Accounts with multiple segments and values for each if needed. In most cases it is enough to have a single segment where you define that as your Item Flexfield and that also becomes your Item Number.
Items are defined to facilitate transactional and stocking capabilities. They also help identify a specific material/goods item that you will transact repeatedly so you do not have to rely on a description to know what was transacted.
The Item Flexfield is like any other key flexfield and can have multiple segments. When you have multiple segments data entry is inhibited and can be cumbersome; however, the multiple segments allow capture of more than one aspect about the item. There is probably an even split between the use of single versus multiple segments in the Item Flexfield. The usage is eased to a large extent with a single segment.