In this case, settings are written using the NFC/RFID ISO/IEC 15693 interface to the virtual registers in FRAM. The ROM library acts on the settings and can execute sampling processes and log the result back to FRAM. The results can then be read out over RF.
Using this method, minimal FRAM usage is required for the application, and more space is available for logging the samples. The main function in this setup is not available to the user. The user can still add ISO/IEC 15693 custom commands to do certain operations as needed. For example source code for this use case, see Section 3.2 and read about the "default" and "SensorHub" projects.
In this case, the user has a main function to develop a custom application with and access is available to all modules of the device. The ROM library support using virtual registers is not functional. ISO/IEC 15693 custom commands are still possible. For example source code for this use case, see Section 3.2 and read about the "NFC" project.
The term virtual registers refers to all the registers that exist in FRAM memory space that are programmable over RF (or a host controller) and control ROM operation.