It’s a so-called “Gentlemen’s agreement”, SSB for above the FCC 40 and AM below. Once you get way below channel 1, 26.5 MHz or so, SSB becomes the dominant mode. There are a few FM channels in there as well, but they seem to be local groups.
26.285 USB is a so-called international call frequency, after a contact is made they QSY off to another frequency. When it gets real busy I’ve heard contact way into 25 MHz.
It all works out very good, but unfortunately there are a lot of Cbers that have a standard AM channels they use, then when it gets busy it’s easy to just flip a switch to go to a quieter channel. So you get the activity on 505 and others.
</p>