Using the Printing Messages(IM:XP)

Inside Macintosh: QuickDraw GX Printing Extensions and Drivers /: Chapter 4 - Printing Messages

Using the Printing Messages
QuickDraw GX sends printing messages to message handlers in the message chain, including the application program, any printing extensions that are active, and the printer driver. You can override any of these messages in your printing extensions and printer drivers to modify or replace the default implementation of the message that is provided by QuickDraw GX. You can also send some of the printing messages yourself to accomplish certain tasks.
You can override printing messages to

manage the job context
manipulate print objects
modify application-initiated actions
write data to spool files
retrieve data from spool files
interact with users through dialog boxes
convert spool files to universal images
send raster image data
send PostScript data
send vector image data
communicate with devices
convert a print record in order to use QuickDraw GX to print documents originally created with QuickDraw
display printing status messages
add menu items to the Finder's Printing menu

The chapters "Printing Extensions" and "Printer Drivers" in this book describe how you can use the printing messages to develop your extensions and drivers. This chapter provides the reference material that you need to understand and use the messages properly.