In the `docker-compose.yml` files, why are certain enumerations done with a dash `-`, and others without?

```
services:
  web:           # the enumeration of [build, ports, volumes, environment] doesn't use a -
    build: .
    ports:             
      - "5000:5000"     # why the - here?  could we remove it and have ports: "5000:5000"?
    volumes:
      - .:/code         # why the - here?
    environment:
      FLASK_ENV: development        # why no - here?
  redis:
    image: "redis:alpine"

```

Another example:

```
version: '2'
services:
   db:
     image: mysql:5.7
     volumes:
       - ./mysql:/var/lib/mysql                # could we remove this - prefix?
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: somewordpress        # no - in this enumeration, why?
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress
   wordpress:
     depends_on:
       - db                            # would it be ok without - ?
     image: wordpress:latest
     volumes:
       - ./wp:/var/www/html            # same
...

```