I believe people confuse “Multicast” sites with “Simulcast” sites, as I did early on. And I now understand the difference as simulcast sites all use the same control (and alternate control) channel frequencies, and multicast sites don’t, each multicast site has their own unique control channel frequencies? I’m looking for clarification. Is this a correct statement?
In terms of a trunked system, a "simulcast" aka "simulcast site" or "simulcast cell" is a collection of two or more physical locations (subsites) that share an
identical set of frequencies. That includes
all frequencies being used in the cell, not just the control channels. Primary (active) control channel, alternate/secondary control channels, traffic channels (voice and data). This collection of subsites form the complete simulcast cell, which appears as a single site # in the system, even though the transmissions are actually coming from multiple physical locations at the same time.
Multicast refers to the transmission of the same talkgroup across dissimilar RF resources. Within a trunked system, this can mean that a talkgroup is being broadcast on two or more sites of a multi-site system. For example, TG 100 might be coming over both a North (Site 1) and South (Site 2) simulcast cell at the same time.
A trunked system can be made up of something as basic as one standalone transmitter site, or one single simulcast cell (multiple transmitters sites on the same frequencies), or any combination of those two including multiple simulcast cells and/or standalone sites. Any transmission that comes across more than one of those disparate RF resources at the same time would be said to be "multicasting".
Multicast can also refer to transmissions that are being broadcast over completely different types of RF resources at the same time. For example, a talkgroup on a digital trunked system could be patched/linked to an analog UHF repeater.
The two terms, simulcast and multicast, can be and often are mixed up. The explanations I've provided are the most common usage when it comes to radio systems.