Subscription Type is a product for which, after the Order has been completed, a subscription will be created and associated. You can choose the product type under the ZOOcart element configuration.
A Subscription product requires a duration, which could be set with a Text/Pro or Select/Radio elements and then mapped for letting know ZOOcart where to get the value from. The value of the element should be a number value, subscription period in days. The mapping can be set on the ZOOcart mapping: Subscription layout under the Type views configuration.
You can override the Subscription duration with Variations if the product has several subscription options.