as per support:
The SQL metadata stream is a relatively new feature (Enhancement). We do not have any public facing documents that describe this in detail. It essentially provides NetBackup with additional data about the SQL environment. It may not be considered a 'critical' part of the backup process, because if the metadata backup stream should fail, you can still perform restores from the database streams that did not fail.
This metadata stream essentially backs up the NetBackup 'history' files. One known issue is described in the TechNote below. This issue can happen if you migrate your SQL backups to a new master server, and the metadata stream will still attempt to run on the old master server.
https://www.veritas.com/content/support/en_US/article.100052792.html