Fireground frequencies are expected to be simplex. At least that’s what NFPA standards call for. Primarily low powered handhelds that work fine on a fireground, not associated with a trunked system or conventional repeaters. They are most likely conventional frequencies, used for fireground and mutual aid work. As such, they may not be a part of a trunked system that requires TG’s or in the case of a conventional system, repeaters. Of course there’s always exceptions, but generally, that’s the way it works.