What is the UART setup on mobile phones?

UART is abbrevation for Universal Asynchronous Receiver/Transmitter.

Dual sim phone usually need to set up UART. You can choose Sim1 or Sim2.

when the phone is dual sim cards,dual standby: if you choose Sim1, then Sim1 may have more functions than Sim 2. For example, some phones do not support dual blueteeth, and at that time only one card can own this function.