The Input Parts button lets you select the input part(s) that you want to be included in the derived part.
For some derived parts, a region is selected by default since that type of part is a logical choice of parent part.
In other cases, the button is marked with empty brackets to indicate that no parts have been selected.
After the selection process, the input parts are named within the brackets. Since space in the button does not suffice for a complete list, you can hover with the mouse to display the information completely.
