You are not confused, they were! The online U3423WE User's Guide page 56 shows the Menu- Input Source. It gives you four video IN choices. This means that you could in theory connect up to four PC to the U3423WE. Correct, when using a second PC connected to U3423WE (DP/HDMI/HDMI), the provided USB-C to USB-A cable is also connected to the U3423WE #8 USB-C upstream data only port, other end to the PC USB-A port.
At the moment I am testing a U3423WE monitor, which has video IN connectors for USB-C (video and data), DP, and two HDMI. I asked the DELL support if it would be ok to connect one notebook to the USB-C (video and data) port and the second notebook to the DP and the USB-C (data only) port. I got the answer, that this isn't allowed. Despite that answer I tested this setup and found no problems. Everything seems to work as expected. Also I couldn't find any hint in the manual of this monitor or any of the other USB-C capable DELL monitors that this isn't allowed. Now I am confused: Is it allowed or is it not allowed and is working only by chance? Thx.
U3423WE and similar USB-C monitors, DP and USB-C in parallel?