This is a snippet from the reference manual. I understand that TRGEN can be set to have an external trigger to start conversion. And the rising or falling edge can be configured. But how is this external trigger source provided? Is it from any of the physical pins or is this some sync pulse they are talking about?
Pay attention to following appnote, section 4:
AN13413: ADC Guidelines Spec and Configuration - Application Note (nxp.com)