Unfortunately, the term "expiration date" has been used with some ambiguity within the various media manager databases.
   
Volume Database:
(As mentioned on page 83 of the 4.5 Media Manager for UNIX System Administration Guide)
Expiration date refers to the age of a volume. If the expiration date is reached, the volume is considered too old to be reliable and Media Manager allows no further mounts
in write mode. Further mounts in read mode are allowed. Using volume expiration is optional, and the absence of a date in the VMQUERY output does not imply that the media is not assigned or that database corruption has occurred.  

Media and Image Databases:
Expiration date as used here is applied to the expiration of a backup image in the image database. This is on a backup by backup basis. In the media database, the expiration date can also be displayed based on the latest backup on a piece of media. The expiration date will be tied to the last date a backup was written plus the retention period. The media will not expire in the media database until all images have expired.