# Zones

> Radar sees people, not furniture. Zones tell Cymo what each part of the room means.

## What a zone is

A zone is a named area of a room, such as a bed, a sofa or a door, with an `id` like `zone_bed`. You draw it once in the dashboard or set it with `PUT /v1/rooms/{room}/zones`, as a polygon in metres in the room's coordinate frame. From then on, every person in the snapshot carries the `zone_ids` they are in. Zones may overlap, so a person can be in several at once.

## Zone types

Each zone has a `type`, such as `bed`, `bathroom` or `doorway`. The type tells Cymo how to read time spent there: time in a bed zone can become a sleep session, and actually getting out of bed is a `bed.exited` event.

## Revisions

Zones carry a `revision`. Send the one you read as `expected_revision` when you replace them; if someone changed the zones in the meantime you get `409 revision_conflict` instead of overwriting their work. Events record the `zone_revision` they were measured against.

## Why zones instead of object events

Cymo cannot see a bed, so it reports facts it can measure, a person entering or leaving an area, and your zones give those facts meaning. The same two events cover someone reaching the front door or a worker leaving their desk.