(Note: The SPI interface pins have been using the problematic terms "Master" and "Slave". That gave us the pin names MOSI (Master Out / Slave In), MISO (Master In / Slave Out), and SS (Slave Select). Sometimes CS (Chip Select) was used in place of SS. Recently there has been some move towards the less problematic terms "Controller" and "Peripheral": CIPO, COPI, and PS.)
The Peripheral Select line is how multiple peripherals share the CIPO and COPI pins. Only the peripheral whose PS line is LOW is allowed to drive the CIPO line and is the only peripheral that acts on the data presented on the COPI line.