The three letters that identify which special device filenames correspond to your installable device driver are stored in a registry key that must be created when your DLL is installed. This three letter prefix is used in many ways by Windows CE. When you create your installable device driver, you must choose this prefix. It can be any three letters.
The prefix serves to identify all 10 possible device filenames that will access your installable device driver. The digit following the prefix in a device file name differentiates between multiple devices managed by the same installable device driver.
The prefix is also used to tell the operating system what entry point names to expect in the DLL for your installable device driver. For example, if you are implementing a device driver for a PC Card pager, you might choose "PGR" as your three letter prefix. That would dictate entry point names such as "PGR_Open" in your installable device driver's DLL. See Writing an Installable Device Driver DLL for details.