related_applications and prefer_related_applications

In one line: related_applications is an array pointing to platform-specific applications related to your web app — each entry needs a platform plus a url and/or an id — and the boolean prefer_related_applications (omitting it behaves the same as false) hints to browsers whether to offer one of those related apps instead of the web app itself.

related_applications

MDN describes it as being “used to specify one or more applications that are related to your web application. These may be platform-specific applications or Progressive Web Apps.” Each entry is an object that “must include a platform property and a url or an id (or both)”:

• platform — a string identifying where the app can be found. Documented examples include amazon (Amazon App Store), play (Google Play Store), windows (Windows Store), and webapp (for a Progressive Web App).
• url (optional) — the URL where the platform-specific application can be found. If omitted, id must be provided.
• id (optional) — the ID representing the application on the specified platform. If omitted, url must be provided.

"related_applications": [
  {
    "platform": "play",
    "url": "https://play.google.com/store/apps/details?id=com.example.app1",
    "id": "com.example.app1"
  },
  {
    "platform": "windows",
    "url": "https://apps.microsoft.com/store/detail/example-app1/9WZDNCRFHVJL"
  }
]

A webapp-platform entry can reference the PWA itself:

"related_applications": [
  { "platform": "webapp", "id": "com.example.app1" }
]

MDN also notes the relationship is unidirectional — “the native apps are not required to reference your web app in return” — and that this data enables Navigator.getInstalledRelatedApps(), letting your web app check whether a platform-specific version, or the web app itself, is already installed on the device.

prefer_related_applications

This boolean “provide[s] a hint to browsers whether to prefer installing native applications specified in the related_applications manifest member over your web application.” Per MDN:

• true — browsers may prompt users to install one of the related_applications entries instead of the web app.
• false or omitted — browsers prefer installing the web app over the related native applications.

{
  "prefer_related_applications": true,
  "related_applications": [
    { "platform": "play", "id": "com.example.hiking-app" }
  ]
}

MDN flags a Chromium-specific behavior: “For Chromium-based browsers, prefer_related_applications should be set to false or omitted to make your web app installable.”

Status

Both members are documented by MDN as Experimental and Limited availability (“not Baseline because it does not work in some of the most widely-used browsers”), per the Manifest Incubations spec.

Practical checklist

• Give every related_applications entry a platform, and at least one of url / id.
• Add a webapp-platform entry referencing your own PWA if you want getInstalledRelatedApps() to detect the web app itself.
• For Chromium-based browsers, MDN recommends leaving prefer_related_applications false (or omitting it) to make your web app installable there.
• Check current browser support before relying on either member in production — MDN marks both experimental with limited availability.

Specifications

Specification	Status
None.