TomWS is correct, in that physically the HCW variant is routed to certain amplifier pins of the module which enables it to reach 20dBm output, but does not allow it to go to the lower range of the CW. That is why there is a HCW (up to 20dBm) and a CW (up to 13dBm).
An observation RE ATC:
I see that when the ATC does actually need to kick in, it is best to go 10 steps up per retry or even up to max, depending on scenario, to ensure delivery. I have thus
modded the lib to allow this.
That is where HCW is more useful to have 7dBm extra power, a max of 100mW output, vs just 20mW of the CW. On the lower end, the HCW is still very low power.