docker-docs/content/includes/compose/services-volumes.md

The volumes attribute define mount host paths or named volumes that are accessible by service containers. You can use volumes to define multiple types of mounts; volume, bind, tmpfs, or npipe.

If the mount is a host path and is only used by a single service, it can be declared as part of the service definition. To reuse a volume across multiple services, a named volume must be declared in the volumes top-level element.