Yep, that's indeed the trick with the Palmetto 800 system, and any multi-site trunked system. Larry can hear SCHP Greenwood because a subscriber radio is affiliated with that talkgroup on the site that he's monitoring, perhaps site #35 Greenville. I can occasionally hear SCHP Greenwood here in Spartanburg on site #19, but not all the time. It's inconsistent.
Some talkgroups are restricted to only being used on one or a few towers. If a radio tries to affiliate with that talkgroup on a site for which it is not authorized, the radio will be denied access to the system and the user will receive an error message. Some talkgroups, but not many, are authorized system-wide. It's not uncommon, for example, to hear traffic on the Meducare talkgroup from Charleston up here in the Upstate. This occurs when an ambulance from Charleston is up here to transport a patient, and you can hear their communications from Charleston while that ambulance is affiliated with the local tower.
The only way to know which talkgroups are consistently active on specific sites is to listen and make notes. Make sure you identify the sites in your scanner program so you can see which site is active and carrying the radio traffic you're hearing. Or limit your scanning to one site at a time.
You can also use a trunked radio system decoding software application like Unitrunker or Trunk88. These programs analyze the control channel data and display site activity in real time. And they also create log files that you can use to determine active talkgroups. This used to be a complicated set-up, but with the new RTL USB dongle tuners available now, you can have this set up and running for $20 or less and not have to dedicate a scanner or add a discriminator tap.
Cool, glad that seems interesting to you. I'm a fan of Trunk88 decoding software. It works directly with the RTL dongle, no other software required, which makes it a little more simple to use. Once you receive the dongle, feel free to hit me up for tips on using it.