In HT notification, task variables can be referenced by expression ${...} and there are some pre-defined task variables which is useful in this feature
as described in old[ BPMS6 User Guide - 27.5.3. Notification|https://access.redhat.com/documentation/en-us/red_hat_jboss_bpm_suite/6.4/html/user_guide/sect_activities_tasks_and_sub_processes#notification].

~~~
Available variables
A notification can reference process variables by using the #

syntax. Similarly, task variables use the ${taskVariable} syntax.

In addition to custom task variables, the notification mechanism can use the following local task variables:

taskId: The internal ID of the User Task instance.
processInstanceId: The internal ID of task’s parent process instance.
workItemId: The internal ID of a work item that created the User Task.
processSessionId: The knowledge session ID of the parent process instance.
owners: A list of users and groups that are potential owners of the User Task.
doc: A map that contains task variables.
~~~

please add the same in 7.x document :
Developing process services in Red Hat Process Automation Manager
CHAPTER 22. CONFIGURING EMAILS IN TASK NOTIFICATION
https://access.redhat.com/documentation/en-us/red_hat_process_automation_manager/7.11/html/developing_process_services_in_red_hat_process_automation_manager/configuring-emails-in-task-notification-proc