Each type of Profibus device has it's own GSD file. And this GSD file has it's own unique identity number.
The GSD file will tell the Profibus master how to talk to the device i.e. what baud rates are available etc. It will also tell the Profibus master what information is available from the device.
This is just a brief summary. For more information you can go to www.profibus.com they have a lot of literature pertaining to the GSD file and the device.