For SPI, this is the Chip Select (which enables the data lines.) Typically LOW to enable -- "CSN" is "Chip Select Not".

Generic GPIO uses PA4.

CHECK: "LPTIM2.OUT/ADC1.9/COMP1 IN- COMP2 IN-"

Devices:
- BME280[SPI] (CS)
- BMP280[SPI] (CS)
- NRF24L01[SPI] (CSN)
- NRF24L01[SPI] (CSN)
- CC1101[SPI] (CSN)