In this mode, you can test flight code on your local machine without MPLAB or Arduino. By substituting the HAL_*.c file with HAL_simulation.c, you can run/simulate the code for CDH down to its I2C implementation on your local machine. This does not require MPLAB and is simply a binary file.
mkdir bin
g++ -D __SIMULATION__ -o bin/test *.c
./bin/test
Rapid CDH uses SAM D51s which requires MPLAB to compile/run. This configuration allows the flight code to use actual hardware I2C implementations.