The Program ID is a unique identifier for your add-on. When you package your add-on for distribution using jpm xpi, it will become the ID field in the add-on's Install Manifest.
The ID is used for a variety of purposes. For example: addons.mozilla.org uses it to distinguish between new add-ons and updates to existing add-ons, and the simple-storage module uses it to figure out which stored data belongs to which add-on.
When you create an XPI with jpm xpi:
- if the package.json does not include an
idfield, then the ID written into the install.rdf is the value of thenamefield prepended with "@". - if the package.json does include an
idfield, and it contains "@", then this is written into the install.rdf as the add-on ID. - if the package.json does include an
idfield, and it does not contain "@", then jpm xpi raises an error and the XPI will not be built.