The Subsidiary DLC allows you to launch an IPO (Initial Public Offering) for your subsidiary company. Through privatization, you could do it the other way round by turning a public company into a private subsidiary company.

To do so, you must first acquire 100% shares of the target company from the Stock Market. Then go to the Financial Actions screen (hotkey: F11) and select the target company. The option Privatization will be available from the list of financial actions. Now click the [Privatize This Company] button to execute it.