When you export folders or assets from the WorldServer Explorer, you can then translate them offline. After you make changes, you import the assets back into WorldServer. This process is different from downloading and uploading assets.
This "ad hoc" process produces a translation kit (also called a project package) of segmented assets. An expiration date noted on the Create Translation Kit for Assets (Step 1 of 2) dialog tells you the earliest possible date that the segmentation data created in the process will be marked for deletion in the WorldServer database.