1) Точно не знаю, но думаю, что RS232_ERRORS для каждого потока своя (имеется ввиду для каждого STREAM=id в директиве #USE RS232)
2) А определения битов из мануала:
The definition of the RS232_ERRORS is as follows:
No UART:
* Bit 7 is 9th bit for 9 bit data mode (get and put).
* Bit 6 set to one indicates a put failed in float high mode.
With a UART:
Used only by get:
* Copy of RCSTA register except:
* Bit 0 is used to indicate a parity error.
Warning:
The PIC UART will shut down on overflow (3 characters received by the hardware
with a GETC() call).
The "ERRORS" option prevents the shutdown by detecting the
condition and resetting the UART.
P.S. Можно пройтись поиском
здесь