So full disclosure, I have never done a deco dive myself so so I'm just trying to absorb the device logic since I own one.
Given my inexperience, I assume if you have another gas set to on, and it would be beneficial in a deco scenario, does the DC alert you that it recommends switching gas?
If you do switch gas, and set that gas as active after you do, does the DC make any incremental adjustments to the displayed metrics as acknowledgement that you took that action?
What if there's 100% O2 staged, but not physically on you? Would it be best practice to have it listed, but not on and not active?
The Teric will display your current gas in yellow (I think it's yellow) when you have ascended to the depth where you could/should change to a richer gas. I.e. once you ascend to the MOD of that gas.
For example, if you tell it you are carrying Air, 50% (MOD of 70 feet) and O2 (MOD of 20 feet), then when you ascend up to 70' while breathing Air, at 70' the Teric will still show that you are breathing Air, but it will show the gas in yellow, alerting you that have a gas programmed that would be better to use from 70 on up. When you change to it, the display of the gas will change to show you're breathing 50% and no longer in yellow.
When you ascend further and get to 20', it will go yellow again, alerting you that you could/should change to your O2.
It does not change any metrics shown as acknowledgement that you switched. The metrics are already being displayed on the assumption that you will switch.
If you have a gas staged, and are planning to use it, then you would have it set to On, even though you are not physically carrying it.
If, for some reason, you staged it but are not planning to use it, then you would have it programmed in but set to Off. You *could* (in that case) choose to not even program it in, because you CAN edit gases during the dive, so you could add it in during the dive, in the eventuality that you did decide to use it.
A more likely scenario - though not really recommended - is that you and a buddy are doing a deco dive together but you each have different deco gases. Say, you have 50% and 100%, but your buddy has 32% and 80%. In that case, you'd program your 50 and 100 and have them On, and you might program in 32 and 80 and have them off - for just in case something happens that results in you sharing your buddy's deco gas.
As I said, it is NOT recommended that you are your buddy have different deco gases. But, in reality, it does happen on occasion.