According to a Wikipedia reference “https://en.wikipedia.org/wiki/S_meter”, which is also in the ham radio world generally understood, a S9 signal at HF is around 50uV, while for VHF and above, this is 5uV.
For a long time this has confused to why such a ruling would be done, as a 50uV / sq metre of signal at HF is still the same signal present at VHF and above should a 50uV / sq metre be induced into the radio signal antenna.
Now as I see things, the word “induced into the radio signal antenna”, is where the difference comes from, between HF and VHF/UHF antenna's being used for radio use.
A full wave length antenna could be said to induced all of the received wave length of the signal at the antenna end of the radio. Should a full wavelength antenna be used as a reference antenna loading, ATU matched naturally, one could say that all of the signal has been induced over the full wavelength.
However here is where the bug comes in, as if the next person antenna is shorter than full wavelength, then by a ratio of length of wire to induce the radio signal into the antenna, a shorter wire would induce less of the intended signal.
Now as things happen, a full wavelength wire has an inductive reactance of around 550ohms, so a 50ohm matched section of wire would be around 10% of the length of a full wavelength antenna, the 50ohm match not requiring the use of an ATU circuit.
Now the rub, as the 50ohm matched antenna is only 10% of a full wavelength, it can be said that only 10% of the 50uV / sq metre of signal is being induced into a 50ohm stub antenna thus presented to the radio receiver front end amplifier, in other words only 5uV of signal, as opposed to the 50uV / sq metre of signal present within the surrounding air.
Now walkie talkies essentially use a 50ohm stub antenna, so their S9 signal projected back into the surrounding air is still 50uV / sq metre of signal, but although only 5uV is used, due to the antenna efficiency. Thus when bench testing a walkie talkie, a test signal into the antenna socket in place of the 50ohm stub antenna for an S9 signal test ( effectively a 50uV / sq metre of signal ) would thus be a 5uV signal from the signal generator, which according to the reg's, is a VHF/UHF S9 signal.
Should your walkie talkie or HF radio set use a 50ohm stub antenna, then a terminal end signal strength of 0.5uV would require a field strength signal of 5uV within the surround air. Thus by comparison, a 1uV terminal signal would require a 10uV / sq metre of signal within the surround air.
So as one can see, a S9 signal is purely relative to the antenna system in use.