The TX and RX Termination settings are chosen by Quartus based on data rate when the “Use default TX PMA analog settings” and “Use default RX PMA analog settings” are enabled. If you use different values, you can use the default settings as a starting point and perform signal integrity analysis to assess your channel performance.
You can compile your Quartus design and inspect the fitter results to determine the default TX and RX Termination settings for your Native PHY variant.