Well, 89.1% (depending on if that is power or voltage) is about 1 percent
or so below FS.

In this context, 100% is referenced to a digital sample value of +/-
32767, which is the maximum level that 16-bit audio can handle, and is
commonly referenced as "full scale" or "0 dB".

Working backwards from this benchmark, 89.1% represents 1.00 dB below
full scale, or "-1.00 dB". That is a good "safety margin" to ensure
that no clipping occurs, however, even as far back as 1986, CDs were
commonly mastered with peak levels reaching exactly full-scale, or
"100%". However, that is only "acceptable" if a SINGLE digital sample
is at the 100% mark. If two or more samples in a row reach full
scale, that is defined to be "clipping", and on some equipment, it
will actually cause a "clip" light to illuminate when that occurs
during recording or playback.

In order to prevent this while still allowing for MASSIVE increases in
loudness, modern "hyper-compressed" CDs typical limit their maximum
level to something like 98% or 99%, or -0.1 or -0.2 dB. That way, you
can have 10 or 20 samples in a row all "slammed" against this
arbitrary limit, resulting in a "hacked-off" waveform that is
essentially a square wave -- but since it's not TECHNICALLY "clipping"
since it doesn't reach the 100% mark, the proper term for this is
"hard-knee limiting" -- which has become so common on CDs within the
past decade that it is the NORM... very few popular music CDs are NOT
subjected to large amounts of "hard-knee limiting" these days.

For more information on this, please see the following
well-illustrated web site:

http://rvcc2.raritanval.edu/ktek9053/cdpage

Note that on this web site, only the term "clipping" is used, because
that's merely what "hard-knee limiting" is an euphemism for (sort of
like putting extra chrome on a Toyota and calling it a "Lexus").