With the Manage Designs toolbox, you have the option of opening multiple designs in their own design tabs.
Select a design or designs holding down Ctrl as you click, and then click Open Selected.
Alternatively, click New From Selected. This command creates a copy which opens in a new design tab, thus preserving the original design file.
You can also open selected artwork by the same methods.
There are other ways of opening designs into your design window. The Standard toolbar which is available at all times, also lets you open designs from the Design Library or any other network location.
The Standard toolbar also allows you to open new blank designs.