You would first include such plans in a promotional plan set, then associate the promotional plan set with a promotion.
Your customers can then enter a promotion code when subscribing to new plans or you can provide a promotion code programmatically in your Account Registration application.
You can use promotion codes to filter the plans that are available to a customer either in your Account Registration application or on the Create Accounts screen. One of the most common reasons why you would use promotion codes is if you work with resellers/channel partners/agents who sell your products or refer users to your sites and receive a commission from the resulting subscriptions.