Hi,
Is there a PDF of the M0 schematic? I have the image from the website, but the small text is hard to see because it's pixelated. For example, pin 9 and 10 say RXI/TXO, or maybe it's RX1/TX0?
It's readable, it's TXO and RXI. Those are just schematic symbols.
Look at the PCB as well, the pins are the same and are marked on the silkscreen. TX/RX is "Serial", TX1/RX1 is Serial1.