Thank you very much for the explanation! <3
Unfortunately this is kinda underwhelming holding in that case :(. I mean +10 nav cap isn't bad, but the mechanic of getting new pops should be buffed. 20% on 5 years is just silly.

@Тривиум Upon building this holding on a subject planet, it sets a hidden event to trigger in 1800 days (5 years), and the event repeats every 1800-1900 days. This event checks if there is a pop on the planet which has Full Citizenship in the overlord's empire, and if this check is passed, then there is a 20% chance that one pop occupying those Recruiter jobs will be resettled to the overlord's capital. You also take a -3 loyalty penalty from the subject whenever this is successful.

So to maximize your chances, make sure you are setting your subject's primary species (or most populous non-slave species) to "full military service" and "full citizenship". Place the holding on a medium-to-high population planet, so as to make sure the jobs are filled. Also this is useless if your subject is a gestalt, as those pops can never have "full citizenship" in their hive-mind form anyway (at best, they will be set to assimilation first), so the event is skipped altogether.

@Тривиум This question might have been posted to the wrong place, but I can help anyway.
For the sake of thoroughness, the self-explanatory modifiers are +2 unity from your soldier jobs and a +15% modifier to your naval capacity. There is also a restriction on species rights where a species must be set to Full Military Service (they can be commanders as well as recruited armies) before they can be given Full Citizenship.

The interesting part that your question concerns is in the recruitment office holding. The holding itself provides +10 naval capacity to the overlord, and 2 "Overlord Recruiter" jobs for the subject on that planet. The job itself doesn't do much, just occupies a couple of your subject's pops and provides a few defense armies on the planet. (continued...)

Dude please explain to me what are the exact mechanics of Citizen service? Can I increase the chance of getting the enemy pops via recruitment offices?

This was already done in "Counter Limited Armies Fix"