explanation about serout2 and serin2

i want to know what does Flow pin mean?

SERIN2 DataPin{\FlowPin},Mode,{ParityLabel,} {Timeout,Label,}[Item...]

in this command i think flow pin in a test pin to out the recieved bits
to return it to transmitter pin at flow pin ??? is that right ???

SEROUT2 DataPin{\FlowPin},Mode,{Pace,} {Timeout,Label,}[Item...]

in this command flow pin recieves bit from the reciever datapin to test it if there is an error or not, is that right ?

This option is for "flow control". It signals the sending unit to stop or send inbound data. It's used to keep inbound data from over flowing, and allows the receiving PIC time to perform houskeeping or move received data.