You can use triggers to send an email to the user when their ticket is assigned (e.g. to reassure them that their problem has come to the attention of a real person).

Here's how you could implement this:

1. Create a new trigger, and set the event to be When a property on a ticket is changed/By an agent.

2. You probably don't want the user to get a notification if their ticket was assigned and replied to at the same time, so deselect Also run when properties have changed during a reply.

3. Add the Criteria: Agent / changed from/ Unassigned.

4. Add the Action: Send User Email > Create new template.  An editor will open where you define the email template.

    Here's an example template:

    Subject:

RE: {{ ticket.subject}}

Body:

{{ phrase('user.emails.greeting') }} <br />       
Your ticket has been assigned to {{ ticket.agent.display_name }}. <br />
{{ phrase('user.emails.ticket_access_ticket_online') }}  
<a href="{{ ticket.link }}">{{ ticket.link }}</a>

5. Save the template and trigger. Make the trigger active with the toggle control next to its name.

Note that this trigger doesn't run when the assigned agent changes, only when the ticket moves from unassigned to assigned. So in most cases, the user would only get the custom email once. But there is a flaw in the trigger: it will also run if the ticket is moved back to unassigned, then assigned to an agent again.

You could refine this trigger so that it only runs the first time the ticket is assigned. One way to implement this would be by using a custom "Has been assigned" agent-only toggle field, which starts out as unchecked.  Add an action to set the field as checked when the trigger runs. Then add to the criteria of the trigger a check that 'Has been assigned' is false. This ensures that the trigger can only run once (unless agents edit the custom field.).