A few general questions about the CODE

Hi All, I have a few general questions:

(1) Ser2mod.pbp, I’m not sure where I found this document but it has some good examples of using Serout2 expressions. At the end of this document there is a bonus note:
To count the number of bytes actually sent in a 25 byte delimited string.
NumberBytes=25-R$.Highbyte
Has anyone used this undocumented system function successfully using it after a STR receive?

NumberBytes var byte

Serin2 PinOut, 396, [STR Mystring\25\”~”]
NumberBytes=25-R$.Highbyte


(2) What is the difference in these commands?
If
Ifdef
Ifndef

(3)
BIT?GOTO
L?GOTO
MOVE?CT
CHKRP?T…………. what does the “?” do……. Are these system functions or customized commands or labels?

(4) Does anyone have a list of undocumented system commands?



Thanks,
Wayne