About Printing Extensions(IM:XP)

Inside Macintosh: QuickDraw GX Printing Extensions and Drivers /: Chapter 2 - Printing Extensions

About Printing Extensions
Printing extensions are add-on software components that you can develop to extend the printing capabilities of QuickDraw GX. Printing extensions are used for tasks such as supporting hardware additions and modifying the appearance of printed pages and allow you to provide these capabilities without having to implement an entire driver.
For example, you could develop an extension that allows users to control a sheet feeder on a printer or that adds a confidential stamp to every page produced by an application.
Just as Macintosh system software extends its functionality with system extensions and control panels, QuickDraw GX allows you to extend the functionality of printing by providing printing extensions. You can

add panels to the QuickDraw GX print dialog boxes
obtain additional information from users to control options within your extension
modify the manner in which each page, or a whole document, is printed
drive hardware extensions to printing devices

This added functionality is completely independent of the mainstream execution of an applications, and users can activate an extension at the time they run an application. Active printing extensions reside in the Extensions folder inside the System Folder, along with system extensions and control panels. Unlike system extensions and control panels, which run at system startup time, printing extensions run at print time. Any number of printing extensions can be active for a given print job.
Application developers can create applications that initiate printing functions without knowing whether printing extensions will run with the application. When a printing extension is active, QuickDraw GX adds the activity of the extension to that of the requested printing function. If no extensions are active, QuickDraw GX just performs
the requested printing function. QuickDraw GX takes care of locating and activating the printing extensions at the appropriate time. All you have to do when you develop the extension is to provide QuickDraw GX with information about how and when you want the extension used.
Since printing extensions run independently of applications, a single extension can run with a number of different applications. Thus, for example, you can write an extension that adds a special stamp to a printed page, and users can activate it whenever they
want that stamp added to their printed documents.
Printing extensions also allow you to write a single piece of code to handle hardware add-on devices rather than rewriting a driver to do it. By doing this, you provide users with the ability to support the hardware add-on for multiple printers. Your extension can add a panel to one of the print dialog boxes to extend control of the device to the user. For example, the same extension can drive a sheet feeder device that you can attach to a number of different printers.