When you want to work on an application there are 2 option for that:
Open application Activity
You always have to indicate screen to robot can detect how application look like(or you should show it where it will look into with selector variable).
Start Process Activity
You can start application with this activity. After that if you want to work with this you should define it again with Attached Window Activity if you keep working on it. This is different from open application.
If you use Start Process Activity, the application will be open until using kill process activity.
If you use Open Application Activity, the application will be closed after activities are done on it.