On what basis child jobs are created?

From the context sensitive help in the Policy Attributes tab:

The Allow multiple data streams attribute specifies that NetBackup can divide automatic backups for each client into multiple jobs. The directives, scripts, or templates in the backup selection list specify whether each job can back up only a part of the backup selection list. Since the jobs are in separate data streams, they can occur concurrently.

• The directives, scripts, or templates in the backup selection list determine the number of streams (backup jobs) that start for each client. The list also determines how the backup selection list is divided into separate streams.

• The following settings determine the number of streams that can run concurrently:

   • Number of available storage units
   • Multiplexing settings
   • Maximum jobs parameters

 

Multistreamed jobs consist of a parent job to perform stream discovery, and children jobs for each stream. In the Activity Monitor, the children jobs display the Job ID of the parent job. Parent jobs display a dash (-) in the Schedule column.

So, the parent job will 'determine' the number of child jobs depending upon what it finds on the client with respect to the policy's backup selection list.

e.g. (with multiple data streams selected in a policy) a backup selection of:

ALL_LOCAL_DRIVES - number of streams would be determined by number of drives (+ system state) or filesystems on client

C:\SOMEFOLDER\* - number of streams would be determined by the number of folders/files directly within the SOMEFOLDER folder

C:\SOMEFOLDER or C:\SOMEFOLDER\ - number of streams determined by backup selection itself i.e. 1

C:\SOMEFOLDER\A and C:\SOMEFOLDER\B - again, number of streams determined by backup selection itself i.e. 2

Hopefully you get the idea - stream discovery by the parent essentially determines the number of streams as a consequence of what it 'discovers' on the client

Catalog Backup is a different matter as Marianne has already mentioned.