profibus IO in step 7

I am a little confused as to how the profibus comunicates with field devices mostly slaves. How the IO knows what the configured hardware does.
Example I have say a Grease metering controller running via profibus it is assigned 3 input words and 3 output words how within these words does it know if I want to tell it what program to run or when to start etc I mean if I call one of these words how do I know what word to call? I guess there must be some kind of map from the manufacture to tell you this? or do you know by looking within the GSD file?
Please someone put me at rest

The manufacturer of the Profibus slave device must provide some kind of manual, and the manual must describe in detail the mapping and the functionality of the i/o.

The GSD file only tells how many bytes or words to exchange, not the functionality of each bit, byte or word.

Is this the same for serial data exchange?
I have looked in many manuals but don't see any type of field bus mapping anyone got any links or examples of this mapping just to see the structure.
Thanks