externally_connectable

"externally_connectable": {
  // List of extension IDs that extension wishes to communicate with
  // If "browser_action" is specified but "browser_action.ids" is not,
  // no extensions can communicate with this extension.
  // To match all extension IDs use "*".
  "ids": [
    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
    "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
    "cccccccccccccccccccccccccccccccc"
  ],
  // Regular expression patterns specifying web pages
  // If "matches" is not specified, no web pages can communicate with
  // this extension
  "matches": [
    "https://example1.com/*",
    "*://*.example2.com/*"
  ]
}

Externally connectable allows extension developer to control which other extensions and web pages can communicate with this extension via runtime.connect() and runtime.sendMessage() message passing. If externally_connectable is not specified, all extensions can communicate with each other but not with web pages.

"ids" attribute

ids enables communication between this extension and other installed extensions specified by extension identifiers. Use the pattern "*" if you wish to communicate with all extensions.

"matches" attribute

matches enables communication between this extension and the web pages. This is a list of regular expressions for page URLs that you wish to communicate with.

Browser compatibility