Loren Coe writes:

Richard is saying that CFM is at 1 atm, right?

CFM, so-called "free air". CFM with regard to compressed air means
cubic feet of free air per minute.

so an ideal compressor
would take in 'X' CFM/min and output that same amount, regardless of
the output pressure.

Any system without leaks must do this, not necessarily an "ideal" one,
since the air can't go anywhere else. Whatever goes in the inlet can
only exit via the delivery fitting, unless there are leaks. Of course,
this is not instantaneous equality, as the air may spend an arbitrary
amount of time in a reservoir tank before leaving for delivery.

also, i gather that SCFM is simply corrected for moisture content, and
some other factors that would not impact comparison of honest ratings?

Right, "S" in "SCFM" means CFM performance under conditions of a
standard temperature and humidity of the input air, namely 68 deg F and
36 percent relative humidity by my citations. There seems to be a
question about whether these actual values are standardized, as people
are citing a variety of purportedly standard values. This condition
applies to the *ambient* conditions, that is, the input air, *not* the
state of the *compressed* air, which unless treated by a cooler-drier
will typically be 100 percent humidity and 100s of deg F even when the
input air is moderately cool and dry.