Different groups have different radio resources, and radio ID tracking on these different systems or channel groups indicate they are different user groups who mainly stick to their own systems or channel groups.
Of the main groups there are:
CBP, better known as Border Patrol - They interface with Charlie 100, and in some areas have a local dispatch center during the day hours (like 220 in Florida). In the Northeast they use the old US Customs frequencies. Florida did a total reorg of frequencies to the new bandplan format, as they were late to go P25. ID usually starts with 5xxxxxx, but can be overlapping with Park Service units that are also in this range. ANY DHS unit can use the CBP network, but ID tracking shows the different groups tending to stay in there own channel plans. There are also guest agencies who are too small to have their own networks that fall into this network. ID numbers are slightly different, but usually somewhat close to the host agency ID range.
Air and Marine (Omaha, Mike Romeo and Mike designations...as well as some Xray air assets) also interface on this network. India units (Inspectors) sometimes interface on this network. Alpha units (HSI agents) rarely, as they have their own ICE networks. CBP also has POE repeaters (Port of Entry) that serve India units at major ports and airports. These have been used for local tactical repeaters when the need is there and they don't want to tie up the main network. And the Office of Air and Marine group mainly uses a special group "AIR" simplex frequencies to talk between air and marine surveillance units near coastal areas. They also have remote fixed base stations to talk to Xray 950 for target intelligence and engagement rules on some of these AIR channels.
ICE, (ID's start with 6xxxxxx) which mainly uses a different network of repeaters, and a separate group of simplex frequencies, is mainly divided into two sub-groups. ICE ERO, the guys who knock down your door at 5:00 AM and send your @$$ back to where you came from, and ICE HSI (Alpha Units) who are investigators for DHS. ID tracking says these two groups don't interface much at all. The HSI guys have extensive new repeater networks broken into region groups with different NAC codes used on each group of repeaters. Although there was a time when encryption OTAR wasn't set up yet in Florida, and most comms were in the clear (a rare treat), they are now mostly encrypted full time. Activity can spike at times, and be quiet for long periods of time. In the Northeast, other agencies like USMS (8xxxxxx) and BATFE (5xxxxx) use this network (although I have found some exclusive USMS NAC 293 frequencies in both SE Florida and NYC/NNJ.) Not really the case in Florida, at least it wasn't when I was there. BATFE also has their own "basic" national repeater pair and simplex channel plan. BATFE was building NAC 650 repeaters in Florida, but up here in NY I have not seen that. DEA VHF was also building out nicely in Florida, but only very few up here in NY so far.
ICE ERO units use a group of simplex frequencies (usually NAC 169...or something close like 069) and also have a few fixed bases out there with cell phone access. Then there are some Detention Center repeater and simplex frequencies. You know it when you hear what activities are occurring in these facilities. While HSI is a sharp group of agents, ERO doesn't have the same status (from what I have heard).
FPS uses their own UHF system, but is equipped with VHF radios, as well. They "could" come over.
Simplex channels for direct comms exist, and differ somewhat in different areas based on local frequency availability. These are hard to catch, and require excellent station conditions (good sensitivity without image interference on good antenna systems). Once again, the radio ID tells the story. Many times I have seen them use the LE channels as alternate simplex channels. This appears to be when they don't want to be heard by management. Those conversations are enlightening, if not humorous, when you catch them. NAC codes can differ or be the same. ID is the only real way to know.
This breakdown of frequency use is based on years of first hand account logging and tracking in Florida and New York/New Jersey (including heavily populated city areas). Even if you have an agency's "code plug" that someone "leaked", you can't tell how they use the channels other than they have access to them. About all you can tell is how they group the zones, and that might surprise some to see how unsophisticated the zone groups and scan lists are laid out...and redundant at times. Actual system usage has to be extensively logged over a large geographic area and large time period to tell the story. You can't do that sitting at home in one place pretending you know what's happening in areas you haven't actually monitored and logged actual radio traffic in. Armchair quarterback from afar don't cut it. You have to put in the monitoring time. No doubt it's time consuming, and requires good multi-scanner model equipment setup to catch different types of system protocols. I enjoy the challenge. Even with all the heavy encryption use, occasionally you catch a tidbit or two that brings back the old days of monitoring glory of these serious agencies. It's the best!