Hello Yuegao,
The trigger option is described in the Chapter 19 – Trigger MUX Control. For the LPSPI0 it is out92 - TRGMUX_LPSPI0 – on Figure 19-3, page 422 of S32K1XX_RM:
https://www.nxp.com/webapp/Download?colCode=S32K1XXRM
So the trigger is the output signal from Trigger MUX.
I hope it will help you in your study.
Best Regards,
Stano.