The following serial interrupts are associated with the specified bits of
the  interrupt registers .

Bit 11, RBF (for receive buffer full), specifies that the input buffer of
the UART has data that is ready to read. This bit generates a
 level 5 interrupt .

Bit 0, TBE (for "transmit buffer empty"), specifies that the output
buffer of the UART needs more data and data can now be written into this
buffer. This bit generates a  level 1 interrupt .