On what basis child jobs are created?

Hi,

We schedule backups and enable the multiple data streams option. 

If we specify three NEW_STREAMS in the backup selection attribute of the policy we will get three child jobs and one master job.

But in some policies we do not specify any NEW_STREAM but multiple data streams is enabled. Then on what basis the child jobs are made\streamed by default of a policy. In some we will see 3 child jobs and in some 4 child jobs. On what basis these are created by netbackup?

Best exmaple Hotcatalog backup. It automatically creates 3 child jobs.  Though multiple data streams is not enabled by default.

Like this some policies create their own. On what basis?