Security Unit unlock after certain main mission

One of the requirements to develop D-Dog's Sneaking Suit (stun) is Security Unit level 20. I am currently on Main Mission 20 and I do not have the security unit yet. When is it unlocked?
I have an option to auto assign staff members prioritizing security unit over combat unit, but this doesn't do anything actually, so my guess is it needs to be unlocked first after completing certain main mission. I tried googling, but was unable to find anything about security unit at all. It was not even listed on wikis and other pages about MGS V - is it a part of combat unit, perhaps?
Best Answer
The Security Team becomes available after Mission 22, which introduces the FOB mechanics. It is, in fact, separate from the Combat Team, though it also uses Combat ability as its rank-criteria statistic.
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How do you unlock the security unit in MGSV?
Unlocking mission 43Focus on completing side-ops - in my case, I had to complete three of them. When you finish a sufficient number of side-ops, you will be called to the Mother Base to listen about an outbreak of a new epidemic. The new mission will appear on the list after a cutscene.How do you unlock Mission 43 in phantom pain?
You won't be able to build your first FOB until you finish Mission 22: Retake The Platform. When you start one of the missions before it, you'll get an urgent call from Mother Base, saying it has been invaded.How do you unlock FOB missions MGSV?
You must have recruited Quiet, max out your bond with her, spare her at the end of Mission 11, and have the scientist exiled from Mother Base. This will unlock Side Op 150 for play. Enter and complete Side Op 150, and Mission 45 will begin immediately at the end of that.Terrorbyte Client Mission 'Collectors Pieces' [After Hours DLC]
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Answer 2
You're very close - it unlocks after Mission 22. That's all I can say without getting into spoiler territory.
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