They are also sometimes called publish on demand or otherwise known as Print on demand (POD). They are a business process and printing technology in which usually “new copies of a book or any other document” are strictly restricted from printing until and unless an order has been obtained. Print on Demand book publishing progressed only once digital printing had began. The primary reason being it was not at all reasonable to print single copies by means of conventional printing technology such as offset printing and letterpress.
Most of the conventional small presses have exchanged their conventional printing equipment with print on demand (POD) equipment. There are many university presses and academic publishers that use POD services in order to retain a significant backlist. There are some who even use POD for almost all of their publications.