To create a workflow that sends a message when a custom field is modified, you must first have custom attributes to operate with.  Please refer to the Alert Templates document to help create custom attributes.  You may extend the Standard Alert or create a new Template.  For this document, we will employ a new Template with an Attribute named Severity, using the example of a Numeric List, where 1=High, 2=Medium, and 3=Low.

Once the attribute is configured, select Workflows from the left-hand menu.  From the Workflows page, press the Add Workflow button.  
On the Workflow Detail page enter a Name for the Workflow.
Select MessageThread for the Type.
Choose the Template you created your custom attribute in.  If you chose to extend the Standard Alert rather than create a new Template, select that one.

Next, press Add Start Condition.  For this example, we will use “Match All Conditions”, but you may use either as needed.  
Select Template in the Attribute field.  This will allow custom Attributes from the Template we chose above to be available for selection in the Name field.  
Select Severity from the Name field.
Leave the operator as “is”.
Select the Value from the list.  We will use “High” as the Value.  
Press the green checkmark.

At the bottom of the page, press Add Action, choose SendMessage:

This will produce a dialog with Message options:

Select a Message Rule to govern the routing of the update message (you may have to create a new Message Rule separately), then choose the recipient with either the pre-defined selections or enter the user or group manually.  For instance, “Send To Original Recipients” will deliver to each User who was initially notified of the alert.  Users or Groups not in the original thread (such as a Managers group) may be designated in the fields below.  You may make multiple selections and entries.

In the Message Text box, you can craft the body of the message using static words and keywords from the list.  Save the changes to the Action, and this will update the Workflow.  Finally, ensure the new Workflow is Enabled (select the button in the top right corner of the page).

To add the Workflow to the Message Rule, select Message Rules from the menu, choose your Message Rule and select the Workflows tab:

Note that the Message Rule for the Action must be independent of the Message Rule this Workflow is applied to (e.g. This Workflow cannot be applied to the “Email msg on alert update” rule, as that rule was chosen to govern the delivery of the update within the Workflow).  Any Message Rule already used in a Workflow cannot have additional Workflows applied, and the Add Workflow button will be greyed out.

Press the Add Workflow button. In the resulting dialog, select your Workflow (it must be enabled to be present here) and press the arrow button to move it to the Selected Workflows list:

Press Update and the new Workflow is applied to the Message Rule and saved.

As we used a new Template for this example, we must ensure that the Template is applied to the Message Rule.  In the Message Rule page, select the Others tab.  In the Thread Template field, select the Template containing the necessary attribute, and Save Changes:

To demonstrate, we will create a new Manual Alert :

Notice the custom fields from the Template are presented when selecting the Message Rule.  When the Alert is created using the High Severity attribute, the Workflow will send an additional message.  In this example, the Managers group will receive a separate notice when the On-call team is alerted.

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