What a focal area is
On a busy project, different specialists often review different parts of the same work. A focal area lets you name one of those areas — for example Health, Safety, Security and Environment — and hand a reviewer responsibility for just that.
A reviewer tied to a focal area is a focal reviewer. You can structure your project so a focal reviewer sees and reviews only the work belonging to their area, not everything in the project — a clean way to split review duties without creating separate projects or over-sharing.
This doesn't happen by itself: focal areas work together with referral workflows, which send the right work to the right focal reviewer. Creating the focal area and assigning the reviewer (this guide) is the first half; setting up the referrals is the second — see What is a workflow? and Refer in the Workflows guides.
You define focal areas once in Project Settings → Users → Focal Area, then assign them when you give someone the Reviewer role.