Hello David,
as you know the UART interface has defined protocol, which consists of:
1. START bit
2.Data array - usually 8-bit (one byte), but could be 9-bit also,
3.Parity bit
4.STOP bit.
So the information is covered in the Data = one byte. It is up to you what these data will represent. It can be "read" as ASCII character, as hex format number (0x..), as decimal number (0 - 255) or something else.
So what is placed into UART Tx buffer, the same will be received from the UART RX buffer on opposite site. The final interpretation depends on the system using the transferred data.
Best Regards,
Stano.