From the Job Management Database Maintenance Guide
manual for CA-7:
DB.2.6 - Data Set Triggering Panel
Use this panel to define or review data sets that trigger job scheduling when
either their creating jobs complete successfully or when the created/updated
data set is closed.
The answer to your question is that the backup job has no idea about when its files are received. CA-7 tracks the data set(s) and gets the backup job running once the data sets are created and closed or the creating job completes successfully.
You would be wise to read this manual cover to cover to learn more about how CA-7 does triggering (and everything else).