Firebase Emulator start with 2 realtime databases?

When starting the emulator I see 2 databases in the realtime database emulator. One with <project id> and the other <project id>-default-rtdb .

Can anybody explain or reference some docs on why this is?

This is my setup files

firebase.json

{
  "database": {
    "rules": "database.rules.json"
  },
  "functions": {
    "predeploy": [
      "npm --prefix \"$RESOURCE_DIR\" run lint",
      "npm --prefix \"$RESOURCE_DIR\" run build"
    ],
    "source": "functions"
  },
  "emulators": {
    "auth": {
      "port": 9099
    },
    "functions": {
      "port": 5001
    },
    "database": {
      "port": 9000
    },
    "pubsub": {
      "port": 8085
    },
    "ui": {
      "enabled": true
    }
  }
}

functions/package.json

{
  "name": "functions",
  "scripts": {
    "lint": "eslint --ext .js,.ts .",
    "build": "tsc",
    "serve": "npm run build && firebase emulators:start --only functions",
    "shell": "npm run build && firebase functions:shell",
    "start": "npm run shell",
    "deploy": "firebase deploy --only functions",
    "logs": "firebase functions:log"
  },
  "engines": {
    "node": "14"
  },
  "main": "lib/index.js",
  "dependencies": {
    "firebase-admin": "^9.8.0",
    "firebase-functions": "^3.14.1"
  },
  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "^3.9.1",
    "@typescript-eslint/parser": "^3.8.0",
    "eslint": "^7.6.0",
    "eslint-config-google": "^0.14.0",
    "eslint-plugin-import": "^2.22.0",
    "firebase-functions-test": "^0.2.0",
    "typescript": "^3.8.0"
  },
  "private": true
}