Looking at the new Duty Officer system, I see that some officers are labelled as being on Active Duty, and others as being On Reserve. If an officer is on reserve, does that mean they cannot be selected as a candidate for a mission? (I.e., do I have to render them all on Active Duty in order to use them?)
Any advice would be much appreciated.

Exact opposite.

Active duty officers (or those in sickbay or restricted to quarters) cannot be used for assignments.

I get it. I'm finally figuring this out.
Thank you for your help, though.

The thing I've not paid attention to is exactly what difference the Doffs make to space and ground stuff. I remember on Tribble a few weeks ago, I sometimes had 12 or 14 different passives going at once (due to the Doffs I think) but I never figured out if that was because of the Doffs I had in my active roster, or because of the mission rewards.

Those passives were tiny buffs you got from some shipboard assignments. Those have been disabled since the skill update. Doffs in your active roster don't show up as buffs, but they do show up in the tooltips of the powers they modify.

remember t only slot DO's with skills to your active duty rosters. dont slot things lthat you have no hope of ever triggering... it just wastes slots in your active roster. like for gorund people who use lots of engineers, slot the various engineers that will beam down bonus turrets/mortars/drones Ect. or add medic bonus to those that use lots of healing abilities.... an added medic would help with healing in some areas.

for space skills... dont slot the transwarp altering DO, that one i believe is currently bugged.

Ah, thanks to both for the great advice. It's certainly something I should be paying way more attention to from the looks of things!

Bit of a shame about the "not showing up as buffs" thing - I quite liked the way that worked.

the skill buffs could make a comeback if Heretic is in the process of changing them over to the new system.

That would be great if it did.