The start of Slave Select Interrupt flag and enable are the SOSIF and SOSIE bits, respectively. The end of Slave Select Interrupt flag and enable are the EOSIF and EOSIE bits, respectively. These interrupts trigger at the leading and trailing edges of the Slave Select input.
The interrupts are active in both Master and Slave mode, and will trigger on transitions of the Slave Select input regardless of which mode the SPI is in. In Master mode, the PPS controls should be used to assign the Slave Select input to the same pin as the Slave Select output, allowing these interrupts to trigger on changes to the Slave Select output.
In Slave mode, changing the SSET bit can trigger these interrupts, as it changes the effective input value of Slave Select.
Both SOSIF and EOSIF must be cleared in software.