The function is based on cashflows. A negative number means Cash PAID. A
positive number means Cash RECEIVED. That rule applies to PV, FV, and PMT.
The general assumption is that you pay cash in (-), wait for a period of
time, then get cash back (+).

Keep in mind that it's possible for you to make a bad investment and have a
negative return.

Look at this scenario:
NPMT 1
PV -$1000
FV $800
PMT 0

RATE = -20%

You invest $1000 for 1 year, make and receive no pmts, and get $800
back...You lost money.

Also, remember that the rate returned is the rate per pmt period. So if
your payments are monthly...you get the rate for one month...not for a year.

Does that help?

--
Regards,
Ron