According to a comment[1]" "An upstream port connects to the host device (PC) while the downstream ports are where you plug in peripheral devices (thumb drives, printers, etc.)."

Of course the 5k monitor does have three USB3 ports. (Not Thunderbolt ports, ostensibly because there isn't enough bandwidth left.)

[1] https://www.cnet.com/forums/discussions/upstream-vs-downstre...