Source - Manual means that the job was started manually from Orchestrator
Source - Agent: Indicates if a job was started manually from the Jobs page, through a schedule or from an Attended Robot. If the job was started by a schedule, then its name is displayed in this column. If the job was started by an Attended Robot, Agent is displayed in this column.
Valuable information indeed but I think the op was looking for which (Orchestrator) user started a job (in my example, admin).
The way you would see it is only via Audit data, where you can notably since as well the source.