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+## Activity Stream Router message format
+
+Field name | Type     | Required | Description | Example / Note
+---        | ---      | ---      | ---         | ---
+`id`       | `string` | Yes | A unique identifier for the message that should not conflict with any other previous message | `ONBOARDING_1`
+`template` | `string` | Yes | An id matching an existing Activity Stream Router template | [See example](https://github.com/mozilla/activity-stream/blob/33669c67c2269078a6d3d6d324fb48175d98f634/system-addon/content-src/message-center/templates/SimpleSnippet.jsx)
+`content` | `object` | Yes | An object containing all variables/props to be rendered in the template. Subset of allowed tags detailed below. | [See example below](#html-subset)
+`bundled` | `integer` | No | The number of messages of the same template this one should be shown with | [See example below](#a-bundled-message-example)
+`order` | `integer` | No | If bundled with other messages of the same template, which order should this one be placed in? Defaults to 0 if no order is desired | [See example below](#a-bundled-message-example)
+`campaign` | `string` | No | Campaign id that the message belongs to | `RustWebAssembly`
+`targeting` | `string` `JEXL` | No | A [JEXL expression](http://normandy.readthedocs.io/en/latest/user/filter_expressions.html#jexl-basics) with all targeting information needed in order to decide if the message is shown | Not yet implemented, [Examples](#targeting-attributes)
+`trigger` | `string` | No | An event or condition upon which the message will be immediately shown. This can be combined with `targeting`. Messages that define a trigger will not be shown during non-trigger-based passive message rotation.
+`trigger.params` | `[string]` | No | A set of hostnames passed down as parameters to the trigger condition. Used to restrict the number of domains where the trigger/message is valid. | [See example below](#trigger-params)
+`trigger.patterns` | `[string]` | No | A set of patterns that match multiple hostnames passed down as parameters to the trigger condition. Used to restrict the number of domains where the trigger/message is valid. | [See example below](#trigger-patterns)
+`frequency` | `object` | No | A definition for frequency cap information for the message
+`frequency.lifetime` | `integer` | No | The maximum number of lifetime impressions for the message.
+`frequency.custom` | `array` | No | An array of frequency cap definition objects including `period`, a time period in milliseconds, and `cap`, a max number of impressions for that period.
+
+### Message example
+```javascript
+{
+  id: "ONBOARDING_1",
+  template: "simple_snippet",
+  content: {
+    title: "Find it faster",
+    body: "Access all of your favorite search engines with a click. Search the whole Web or just one website from the search box."
+  },
+  targeting: "usesFirefoxSync && !addonsInfo.addons['activity-stream@mozilla.org']",
+  frequency: {
+    lifetime: 20,
+    custom: [{period: 86400000, cap: 5}, {period: 3600000, cap: 1}]
+  }
+}
+```
+
+### A Bundled Message example
+The following 2 messages have a `bundled` property, indicating that they should be shown together, since they have the same template. The number `2` indicates that this message should be shown in a bundle of 2 messages of the same template. The order property defines that ONBOARDING_2 should be shown after ONBOARDING_3 in the bundle.
+```javascript
+{
+  id: "ONBOARDING_2",
+  template: "onboarding",
+  bundled: 2,
+  order: 2,
+  content: {
+    title: "Private Browsing",
+    body: "Browse by yourself. Private Browsing with Tracking Protection blocks online trackers that follow you around the web."
+  },
+  targeting: "",
+  trigger: "firstRun"
+}
+{
+  id: "ONBOARDING_3",
+  template: "onboarding",
+  bundled: 2,
+  order: 1,
+  content: {
+    title: "Find it faster",
+    body: "Access all of your favorite search engines with a click. Search the whole Web or just one website from the search box."
+  },
+  targeting: "",
+  trigger: "firstRun"
+}
+```
+
+### HTML subset
+The following tags are allowed in the content of the snippet: `i, b, u, strong, em, br`.
+
+Links cannot be rendered using regular anchor tags because [Fluent does not allow for href attributes](https://github.com/projectfluent/fluent.js/blob/a03d3aa833660f8c620738b26c80e46b1a4edb05/fluent-dom/src/overlay.js#L13). They will be wrapped in custom tags, for example `<cta>link</cta>` and the url will be provided as part of the payload:
+```
+{
+  "id": "7899",
+  "content": {
+    "text": "Use the CMD (CTRL) + T keyboard shortcut to <cta>open a new tab quickly!</cta>",
+    "links": {
+      "cta": {
+        "url": "https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly"
+      }
+    }
+  }
+}
+```
+If a tag that is not on the allowed is used, the text content will be extracted and displayed.
+
+Grouping multiple allowed elements is not possible, only the first level will be used: `<u><b>text</b></u>` will be interpreted as `<u>text</u>`.
+
+### Trigger params
+A set of hostnames that need to exactly match the location of the selected tab in order for the trigger to execute.
+```
+["github.com", "wwww.github.com"]
+```
+More examples in the [CFRMessageProvider](https://github.com/mozilla/activity-stream/blob/e76ce12fbaaac1182aa492b84fc038f78c3acc33/lib/CFRMessageProvider.jsm#L40-L47).
+
+### Trigger patterns
+A set of patterns that can match multiple hostnames. When the location of the selected tab matches one of the patterns it can execute a trigger.
+```
+["*://*.github.com"] // can match `github.com` but also match `https://gist.github.com/`
+```
+More [MatchPattern examples](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Match_patterns#Examples).
+
+### Targeting attributes
+(This section has moved to [targeting-attributes.md](../docs/targeting-attributes.md)).