Once upon a time S units had a specific meaning as part of the RST
system.

S1 Barely perceptible
S2 Very weak
S3 Weak
S4 Fair
S5 Fair to good
S6 Good
S7 Moderately strong
S8 Strong
S9 Extremely strong.

But meter readings, with the exception of a few professional receivers,
have no real meaning other than one signal is stronger than the other.

Some receivers have "Scotch" S meters which read lower than people would
like ; but most modern consumer equipment has sensitive meters which
show even weak signals as the upper end of the scale and thus make the
receiver performance look better.

Then, of course you need higher numbers because even a fair signal is
S9. Solution? add more numbers - usually as dB above S9.

The other problem is that meter deflection is also affected by antenna
gain or loss. Hook up a long wire and that S2 becomes S8 - along with a
lot more noise.

The end result is that while an S meter may show the relative signal
strength on a specific receiver with a specific antenna, they have no
relevance to the readings on a different receiver and/or antenna or to
the true strength of the signal. Which is easily demonstrated:

WWV on 15MHz is currently S5, S7, S2 and S9 on four receivers here.
CHU on 3330kHz is currently S9+10dB, S1, S7, S4 on the same four
receivers.
All receivers are similar performance in terms of sensitivy and
bandwidth, but the HF antennas are all different.

So as a measure of signal strength, antenna and receiver performance S
units are largely irellevant. They are only useful as a measure of
relative strength of different signals on a specific antenna/receiver
combination.

Dave