In full duplex mode SPI master device simultaneously transmits data to a slave and receives data from a slave. This way only a single slave device can be engaged at one time. An SS line is used to select the particular SPI slave device.
In half duplex single read mode SPI master device only receives data from a slave. This way only a single slave device can be engaged at one time. An SS line is used to select the slave device.
In half duplex single write mode SPI master device only transmits data to a slave, and doesn't receive any data from the slave. This way simultaneous operation with several slave devices is possible.