serial.send(anything but null or empty string) will clear the ENTIRE RX buffer as well.
This is equivalent to 1-way RS-232 actions.

serial.receive() and serial.match() only remove characters as requested from the buffer either a specific count (up to 512) or the string returned by match.

FYI, The RX buffer size for each port is 1024 bytes but the max receive length is 512 bytes.

Just to be clear:

If I send a command to port #1, I expect it to be cleared. Will the send command clear only port #1 or will ALL RX buffers be cleared?

It will clear only Port #1 or Port #2 depending on which port. However, please note that RXF9600 Port #3 and Port #4 share a single UART (see wikipedia or google) and therefore a send to either Port #3 or Port #4 will clear the single buffer for both ports.

Lyndel,

The tandem clearing ports of 3&4 part bugs me.

This implies that it is very risky to use ports #3 or #4 for two-way. Prior to sending anything one must make sure that both RX buffers are empty, and hope that there is no unsolicited RX traffic while sending a command on either port.

For example, will unsolicited RX on port #3 clear unread RX data waiting from port #4? Even if I poll every 100msec, a lot of mischief can happen between polls.

That is correct. Therefore, all 1-way stuff is best suited for ports 3 & 4 with 2-way on Ports 1 & 2.