Mapping of Real-Time Process Data in PROFINET
Unless you are a member of Profibus/Profinet organization, you will get exactly zero information on that purpose.
EDIT:
I do use PROFINET every day. While ETHERCAT has all mechanisms  documented open source, the PROFINET doesn't. For the end user it is not important how this initial handshake takes place. ¸
The slave device is configured with a list of received and sent bytes, that you can configure in editor. A GSMDL file has the options about how many modules (byte sequences) a device can have.

Figure 1: A PROFINET device, cyclic data exchange configuration, catalog with possible modules - for this device only one module with 4 bytes output is possible.

Figure 2: A module RES-5010 has a default submodule Communication with 6 bytes in, 4 bytes out. However you can add additional submodules (yet some bytes of other information)

Figure 3: An absolute encoder with telegram options as submodules. Only one telegram can be configured, no additional submodules are possible.
If the device is capable of acyclic transfer, then you can change device parameters in the PLC program if needed. Many new devices have this possibility.

For example the device from Figure 3 (encoder) has also parameters that are sent upon initialization. These are usually fixed parameters tailored for that specific application, but if needed they can be changed by acyclic transfer from PLC program.
Conclusion:
At the power up, the PROFINET master sends the configuration of the cyclic data exchange and device parameters (if any). After completion of the startup, the cyclic data exchange takse place.
If for any reason a device parameter has to be changed or some debug information is required, it can be done by using a acyclic read/write. The acyclic exchange is useful for very large data transfer (one by one) that have no impact on real-time response. Meaningful real-time data shall be transferred within cyclic data exchange.