Tape expiry date is when the tape is going to expire.  Once the tape expires all the images in the tape will also expire.

Image expiry date is when the images on the tapes expire but the tape expiry is still not met.  This case will occur when muntiple retention is enable on a single tape.

Hope this helps

Basically this only applies in multiple retention policies per tape. If you have it enabled, it means that images will be writtent o the tape until its literally full, no matter what date it will be expired on. In this case you may want to expire certain images (especially if you find out you backed up something that would be SOX compliance and you didnt want it in scope) so that you could wipe it out from being restored without an import. Similarly, you can expire an entire tape which allows you to now write back over the entire tape (whether or not the images are there).

Just remember, expire doesn't mean deletion. Deletion only occurs when you write over it, or you run a full erase.

Nitesh,

Why did you cast a negative vote for me.  What did you found wrong in my statement?

Here is how I'd answer the question.
A tape is a media with sequencial writting. While new images are added at the end of a tape, images that are located at or nearby the begging of the tape might expire. In other words a tape that is listed in EMM may have expired images at its beginning.  So once the tape is full, it is not returned in SCRATCH pool till the last image on it expires. 

From the context sensitive help in Media & Device Manager/Media section in GUI (my words in [ ] )  :

"Data expiration [or image expiration]: the date when the backups [images] on the volume [tape] expire"

"Volume expiration [or tape expiration]: Indicates the age of the volume [tape]. If the volume expiration date is reached, the volume is considered too old to be reliable and no further mounts in write mode are allowed. NetBackup can mount the volume in read mode but writes a message to the system application log that the expiration date has been reached. If the column is blank, the volume has no expiration date. Note: This is the expiration date for the media itself not the expiration of any images stored on the media"

The former is determined by the retention period of the save(s) written to the tape, the latter is or can be set manually by the NB Administrator by right-clicking on the volume in question & selecting change.

Thanks to all

I am curious why certain ones of us are getting negative votes on this. There is nothing wrong with our statements. And its related to the material. Thats just annoying.