Media Error Threshold

the default time window of 12 hours for tapes to be frozen or drives to be downed - are you expecting to exceed this?

If you constantly do get frozen media/downed tapes then I would seriously suspect the resources (tape/drive).

Unfortunately can't comment on what counts as a soft error in BE - but if it constitutes a read or write error then I would imagine that it will count towards the threshold - but again, only within that time window.

***EDIT***
I'm presuming you've already referenced the following:

DOCUMENTATION: How Veritas NetBackup determines if a tape should be frozen or the status of a tape drive should be changed to down, and how to change this behavior
http://www.symantec.com/business/support/index?page=content&id=TECH66391

This T/N references the NB6.0 Media Manager Guide which provides a list of tape alert codes & what the default actions for these are (e.g. freeze media, down drive etc) which may also assist:

http://www.symantec.com/business/support/index?page=content&id=TECH43934