Newly issued securities that have not seen much interest, or subscriptions, from investors ahead of the issue date or have not been offered by brokerages. If you wanted to own the newly issued shares, you'd only be able to purchase them as you would any other stock - through the secondary markets.

Essentially, you can view a subscription to a public offering as an order to purchase the shares from your brokerage firm once they are issued. If you are not subscribed to a given public issue, you will not be buying any shares through the public offering.

The investment bank handling a public offering tries to determine which offering price will result in an optimal number of subscriptions for the issue. Too high an offering price is likely to result in the shares being unsubscribed.