Just in case it helps, Matthijs nicely explained:
The SS (slave select) connection is a bit more flexible. On the SPI slave side (the transceiver), this must be connect to the pin (typically) labeled NSS. On the SPI master (Arduino) side, this pin can connect to any I/O pin. Most Arduinos also have a pin labeled “SS”, but this is only relevant when the Arduino works as an SPI slave, which is not the case here.