The STM8S Firmware Library documentation for the command UART3_ReceiveData8() states that the return value for this function is a u8 in the function prototype, but lower down in the documentation its a u16.

u8 UART3_ReceiveData8  ( void )   

Returns the most recent received data by the UART3 peripheral. 

Full description:
Returns the most recent received data by the UART3 peripheral. 
Return values:
 u16  Received Data

Required preconditions:
UART3_Cmd(ENABLE); 

In my function UART3_RX_IRQHandler(void) interrupt 21, if i write the following (below) for a STM8S207, the received_data variable does not always correctly get the expected value.

    u8 recevied_data = UART3_ReceiveData8();

Reading some UART example code, I found the following which works (clears the MSB)

    u8 recevied_data = UART3_ReceiveData8() & 0x7F;

Why are we required to clear the MSB of the received data from the UART?

Thanks for your help :)