If it's a Harris system then any/all of those can be a control channel at any given time. You'll need to program them all into your scanner, and it will follow both the talkgroups and control channels as they change. Only basing this guess on the following wiki: Control Channel Only - The RadioReference Wiki
Guessing that RR just lists all channels as alternate to indicate you'll need to program them all, as there is no primary available to program a single frequency.
The DIA system is indeed a Harris system, replacing their old EDACS trunked system.
As n0esc has already noted, for Harris systems, any of the frequencies can be used as the control channel, and they do rotate through any or all of them over time. While you can set the system as control channel only, you do heed to program every one of the listed frequencies.
855.7125 is the current control channel. The system doesn't seem to have the range as other trunk systems in area. There aren't any mountains or 22 story buildings to stick an antenna on near the airport.
Are DIA still on P25 Phase 1? I see I can't get them up north. Is there anything I can get up north by Ft. Collins that is around Denver like police, fires, ems, ect. I have pro 106 and 197 inside antenna 800MHz on back of portable scanner and telscoping antennas on the bases. abqscan my brother & i have bought pro 96 & 2096 and pro 106 & 197 from you years ago I think you were at Radio Shack around 16th mall. We both still have our scanners.