Training plans or programs are a list of workouts. They can be in a set schedule or just a collection of workouts like "Ab Workouts"
Yes, if you set the status to private, the training plan won't be shown on the training plans tab. You can send these plans to clients via email link which they can open up in the app and enroll into the program.
Yes, training plans are essentially challenges and you can see many customers are enrolled in them.