It is single sideband noise figure. Recent versions (later
IC446 hotfix versions, and IC50XX versions) now allow
you to plot NFieee (IEEE single sideband noise figure),
and NFdsb (double sideband noise figure) as well.

The reference sideband is the sideband where your input signal is.
This is so that it can figure out the signal conversion gain from the input to the output (remember that noise
factor represents the ratio of signal/noise at the input
to signal/noise at the output). In fact spectre computes
it by finding the ratio of total output noise (less the load
noise), to the contribution of the input noise source at
the output - the signal cancels out in the calculation. However, it still needs to know which conversion gain
to use.

So, the input frequency would be found by:

inpFreq = outFreq + refSideBand*PSSfund

so if you're looking at noise at 100Mhz, and have a
PSSfund of 1GHz, then refSideBand of -1 means that the
desired input frequency is -900MHz. If it is 1, then the
desired input frequency would be 1100MHz. If it is 0, then
there is no frequency conversion between input and
output, i.e. 100MHz input frequency.

Later versions also allow you to select the frequency
from a listbox rather than having to work out and type in
the reference sideband.

Note that if you have multiple large signal inputs, then it
is unlikely to be +/- 1 any more since this is all in relation
to the PSS fundamental; +/- 1 works if the PSS fundamental is your LO frequency.

Regards,

Andrew.