As well as the remote device concept mentioned by CraigV, any drive connections that operate at the user level instead of the service level cannot be used by Backup Exec
This usually applies to mapped drives over CIFS (or similar) AND does not usually apply to iSCSI connections to remote storage (or similar) to provide some examples
Looking at SUBST itself, it looks like it can be used to connect to both local and remote devices and based on internet recommendations for making it persistent (where suggestions of using user logon profiles appear to exist) it appears to operate within the context of the user
Whether or not your condition relates to the user context, it is likely that if SUBST can connect to either local or remote resources that we may not be able to code to understand which you have used and therefore block it all.