SparkFun GPS-RTK Dead Reckoning pHAT for Raspberry Pi

I would like to send corrections from raspberry pi to hat.

You pi configuration instructions mention /dev/ttyS0 and /dev/serial0

Which one do I write the rtcm corrections to?

Also I would like this baudrate to be 115200.

In the u-center program, which port would I set to 115200?

Thanks!