Lots of questions to answer here.
A particular site will only carry the audio of talkgroups that have radios affiliated to that talkgroup on that site. A radio could be a mobile or portable radio, or a radio dispatch console. So, if Columbia FD talkgroup is carried on the Sumter site, it's because a radio is affiliated to that talkgroup on the Sumter site. Is it possible that a radio console in the Sumter dispatch center is monitoring Columbia FD for mutual aid purposes?
It's worth noting that agencies pay service fees that are, in part, based on the number of sites their talkgroups are authorized on. So, it's possible (if not likely) that Sumter County only pays for their talkgroups to be carried on the Sumter site and no others. This is why many agencies have a dedicated "Roam" talkgroup that's authorized regionally or statewide, and they only pay for that access on one talkgroup. So you might be a radio use from Sumter and you roam to the Richland site and the system prevents you from affiliating due to lack of authorization.
Bottom line, you as a passive listener have no control over what talkgroups are carried on a particular site.
Charleston is a different situation from other counties. In simple terms, Charleston maintains their own infrastructure and frequencies, but they share the state "core controller." For Charleston County, this means they maintain control and responsibility for their sites and radios and talkgroups, as though it's a completed separate system (like in Horry or York County). But because they use the state core controller, the SysID is the same as Palmetto 800.
So the sites marked as Charleston (County) (sites 10, 11, 12 and 13) will (in general) only carry Charleston County talkgroups. They will not allow other system users to share their infrastructure and capacity.
Therefore, Palmetto 800 operates separate but overlapping sites (9, 15, 18) to serve non-Charleston County agencies. You (in general) won't hear Charleston County talkgroups on these sites.
From a scanning standpoint, treat the Charleston County sites as a separate system. The same is true with Beaufort County. Site 63 is the Beaufort County site, and sites 60 and 61 are the "state" sites.
Other counties allow other agencies to share their infrastructure and capacity. Florence County is an example. Other counties allow Palmetto 800 to own, control and maintain their counties's sites. Anderson and Sumter are examples. This is why the Palmetto 800 Effingham, Sumter and Anderson sites "disappeared" after the cutovers in the last few months.
I hope this helps explain some things.
