If a product is a subscription, you should set Enable Subscription Mode to "Enabled" in Vendor Control Panel > Manage Products > Subscription Settings.
You can setup a trial period or a setup fee for the product. If you are offering a free trial period, you should input zero to Setup Fee/Trial Fee, and input a trial period.
Subscription products can only be purchased as a single item. In the shopping cart link of a subscription product, you should include "clear_cart=true" otherwise the product could be removed from shopping cart if there are any other items in the cart. For that reason, you can not offer bundle discounts and volume discounts with a subscription product to your customers.