The function assigns which core will be running the BLE Component (Either controller and host/profiles in one core, or controller in CM0+ and host/profiles in CM4).
The core running the controller must be assigned the BLESS Interrupt. That means in dual core mode, CM0+ must have the interrupt mapped. If the core running the controller doesn't have the BLESS Interrupt, an error should show up telling you to configure it. Please only assign it to the core running the controller.
You can still assign other interrupts (for example PWM) to the core of your choosing. What the datasheet is trying to say is that the core running the Host/Profiles (in dual core mode the CM4) is going to receive/send the BLE data. So if you have other components that need this information, they should be assigned to the CM4 core. (For example: running on dual core mode, you receive a command from the UART component to send something via Bluetooth. It would be easier for the UART component to run on the CM4 core so that the data could be immediately sent).
This is only a recommendation, you can run everything off the CM0+ and share data to the CM4 with IPC or other methods. Also vice-versa.