# The Program ID # The Program ID is a unique identifier for your add-on. When you package your add-on for distribution using `cfx xpi`, it will become the [ID field in the add-on's Install Manifest](https://developer.mozilla.org/en/install.rdf#id). The ID is used for a variety of purposes. For example: [addons.mozilla.org](http://addons.mozilla.org) uses it to distinguish between new add-ons and updates to existing add-ons, and the [`simple-storage`](modules/sdk/simple-storage.html) module uses it to figure out which stored data belongs to which add-on. It is read from the `id` key in your add-on's [`package.json`](dev-guide/package-spec.html) file. `cfx init` does not create this key, so if you don't set it yourself, the first time you execute `cfx run` or `cfx xpi`, then `cfx` will create an ID for you, and will show a message like this:

  No 'id' in package.json: creating a new ID for you.
  package.json modified: please re-run 'cfx run'

The ID generated by `cfx` in this way is a randomly-generated string, but you can define your own ID by editing the `package.json` file directly. In particular, you can use the `extensionname@example.org` format described in the [Install Manifest documentation](https://developer.mozilla.org/en/install.rdf#id). However, you can't use the [GUID-style](https://developer.mozilla.org/en/Generating_GUIDs) format.