The Start component is the trigger of a Job. There can be several Start components per Job design if there are several flows running in parallel. But for one flow and its connected subflows, only one component can be the Start component.
Drop a component to the design workspace, all possible start components take a distinctive bright green background color. Notice that most of the components, can be Start components.
Only components which do not make sense to trigger a flow, will not be proposed as Start components, such as the tMap component for example.
To distinguish which component is to be the Start component of your Job, identify the main flow and the secondary flows of your Job.
The main flow should be the one connecting a component to the next component using a Row type link. The Start component is then automatically set on the first component of the main flow (icon with green background).
The secondary flows are also connected using a Row-type link which is then called Lookup row on the design workspace to distinguish it from the main flow. This Lookup flow is used to enrich the main flow with more data.
Be aware that you can change the Start component hence the main flow by changing a main Row into a Lookup Row, simply through a right-click the row to be changed.