There are a couple of ways the word "zone" is used.
In this context of radios being used by the personnel in the field, a zone refers to a group of channels that is programmed into the radio. It is usually done in groups of 16 channels that can then be accessed using the channel knob at the top of the radio. Zones are usually labelled alphabetically to help locate them quickly. The A zone would be programmed with that radio users most used channels, so EPSO's A1 is probably EPSO 1 and CSFD's A1 is probably CSFD Fire 1.
The C Zone is a common zone agreed upon between all the El Paso County agencies to program in exactly the same way so that anyone with a radio in El Paso County will be able to talk to each other using the same channels in case of a big event or incident. If any agency wants to talk with each other and they agree to switch to Zone C Channel 2, they will all end up on the same C2 channel.
CSPD is completely encrypted and cannot be heard by a scanner unless they switch to a channel/talkgroup that is not encrypted. The C Zone is not encrypted so that everyone will be able to talk to each other without any issues because not every radio is setup to listen to encrypted radio traffic.
A separate usage of the word zone is more technical for the radio system, which I have used in a previous thread of yours. There are different geographical regions that have separate zone controllers. These zone controllers manage the radio activity for that system's zone, so it figures out stuff like talkgroup routing to all the different sites within that zone. This is displayed in the RadioReference Database as the RFSS field for the different sites for DTRS, which most people will just prepend to the site numbers. El Paso County operates its own zone (Zone 4) mostly in isolation from the rest of DTRS, which is what led to your prior issues with hearing El Paso County Sheriff.
For simulcast, you may or may not need a SDS scanner based on your personal experiences with encountering
simulcast distortion (link). The brief explanation for simulcast distortion is that your scanner gets confused by overlapping radio signals from many different radio transmission sites that are behaving as a single logical radio site for the system and all share the same frequencies. This can lead to broken audio or mysterious periods of silence in combination with your scanner's screen flashing in confusion, even though the actual professional radio users can obviously hear each other without trouble. If your use case is stationary and you can hear radio transmissions fine, then you are lucky and may not need an SDS scanner.
The Colorado Springs Simulcast site is the primary site for all of El Paso County, so almost every single radio user in the county uses that radio site to communicate. The other sites in the El Paso County zone are used as filler sites for the gaps in coverage, so there will be occasions where you will not hear traffic on those sites unless there is someone listening from within that site's coverage area. This is due to a trunking concept called
affiliation (link) which you can read about in the linked page.
