In numerous tutorials on the web they seem to be interchangeable. Is there any important difference I should know about?

EDIT: OK I found in tutorial i02 the following for the [rfft~]:

"forward real FFT. Like "fft~", but only one inlet (for the real part) and only the first half of the output signals are used. (The others are determined by symmetry: they're complex conjugates of the first half, in reverse order.) This takes 1/2 the CPU time of "fft"."

Can anyone explain the importance of this to me? I don't really understand what Miller is saying.

Not sure if I can explain, or even if I have understood it right, but I think that [rfft~] is designed for "real" signals, cause [fft~] creates a mirror of the signal analysis above the nyquist frequency, or something like that... You'll usually see [rfft~] in action more than [fft~].

@rjp9 Why this is posted on off-topic? This is most certainly an "technical issue" post as it's about how Pd or some of its objects work. I am moving this to that sub-forum.