WARNING!
Selections make the most sense when applied to contact-related (C) jobs. If you apply a selection to a system-related (S) job, the selection applies to the event, not the job. The selection then filters the system user who initiated the job. For example, you create a selection that includes all API users in your system. Applying this selection in the event-based job window restricts the automation to those events performed by the API user. In contrast to using a selection for (C) jobs, it does not limit the application of jobs to the API users!