Design Units Tab:
The Design Units tab displays the design units in the project. The Design Units folder is empty until you compile the design. After you compile the design, the design units appear beneath the Design Units folder, and you can click the + icon beside any design unit to expand it and view the name and icon of the corresponding source file. You can also open the source file from this tab.
The Design Units tab displays updates when you compile the design. You can identify the type of each design unit by the text that appears on the design unit icon.