Image Cleanup/ Retention Period

Hi

We are using Symantec Netbackup 7.5 on our Redhat Linux Servers.  In Activity Monitor before any backup process start it always shows Image Cleanup, does that means Symantec is deleting any expired images from disk or tape.

If I set retention period for a particular backup policies as 6 months. Does that mean those particular backed up files get expired after 6 months or is there any way to still access those expired images.