Drawing power for the USB and UART ports
The 5V power from the USB and UART and the main power to RVR come from the same 7.4V battery. There is an e-fuse on the expansion port, which limits the output current to 2.1A.
The USB and UART share the same power supply, so both of them combined will be limited to 2.1A
Can I use the USB 5V just for 5V power?
Yes you can!
RVR does not negotiate power requirements with the device plugged in. You can draw power without communication.
Are the UART rx/tx lines 3.3V or 5V?
The UART lines are 3.3V.