The Item Tab item type is very different from the other item types. What it does is, it inherits another item types' fields and renders them in a tab. The item tab makes is easy to manage a lot of configurability options and divide them into different categories/tabs.
So, in the parameters section (Figure 1.1), you must select the item type whose fields should be rendered in the tab.
A prime example of using the item tabs is the website settings for a Rapido solution (as of this date) shown in Figure 1.2. All these categorized settings are rendered with the Item Tab item type (Layout, Product list, Product Page, etc.).
If we take a look at the item type coupled to the website settings, we'll therefore just see six Item Tab item fields - corresponding to each of the six tabs (Figure 1.3). Each item tab field then refers to another item type (for instance here the Layout item type placed elsewhere in the Item type tree.
And it is from these referred-to item types all the fields and settings are inherited.