Apologies if this is answered elsewhere, but where do certain DOffs come from? It seems I have Colonists magically appear in my roster from time to time.

Same thing for Prisoners. Every now and then I see an assignment to debrief a Prisoner, but do I have to arrest him first? And if so where/how?

Colonists come from specific missions - "Relocate Colonists," I think it is. You need to send out two or three BOffs to get them (I think one needs to be a counselor? Or at least, it helps to get a crit success or something)

I have no idea where prisoners come from; I think I got one on one of my toons recently that came from a standard DOff recruiting mission.

Colonists come from the "Resettle Colonists" assignment, which can be started once every 28h. (4h duration + 24h cooldown). The assignment grants 5 colonists.

Prisoners come from a number of assignments, depending on faction. They are a lot more available to KDF than they are to the Federation.

For the federation, you can usually receive prisoners only on critical successes. They come from teh following assignments:
-Uncover Intelligence Leak (critical only, rewards 1)
-Military Offensive on {Planet} (critical only, rewards 3)
-Raid Safehouse of Facility 4028 Fugitives (5/5) (Rewards blue Dominion Fugitive (prisoner), can reward regular prisoners on critical).
-Strike Against Fugitive Support Network (Critical rewards blue Dominion Fugitive (prisoner) and 2 regular prisoners. Not sure if regular prisoners are possible as a normal success result, repeatable unlocked by above)

Thanks for the info. I thought "Resettle Colonists" sent away Colonists (to go live on their new world). In fact I know that happens. But then at some point 5 more reappear. My question Is, is that automatic? Sounds like you're saying yes, and they will just recruit 5 more after a certain period of time.

Yeah I think I had a Prisoner only once.

No. Resettle colonists picks up colonists. Relocate colonists drops them off. Minor difference in the name, but totally opposite in what the assignments do. It should say what it does in the description if you read it. The other quick way to tell is to look at the rewards before you start the assignment. For 'Resettle', you will see 5 'place-holder' icons as part of the rewards. That means you will be picking up 5 colonists. For 'Relocate', you will be slotting colonists into the assignment slots, and the description will say 'these colonists will be permanently lost' or something to that effect.

I just snagged one Resettle assignment in Eta space, i seem to find them on a daily basis both ways, not found a prisoner yet

Ah, sho 'nuff. Thanks.