In Slave mode, the transfer counter will still decrement as data is shifted in and out of the SPI module, but it will not control data transfers. The BMODE bit along with the transfer counter is used to determine when the device should look for Slave Select faults.
When BMODE = 0, the SSFLT bit will be set if Slave Select transitions from its Active to
Inactive state during bytes of data, or if it transitions before the last bit sent
during the final byte (if SPIxTWIDTH ≠ 0).
When BMODE = 1, the SSFLT bit will be set if Slave Select
transitions from its Active to Inactive state before the final bit of each individual
transfer is completed.