You can send a custom message to assignees when the process management status is changed.

By default, notifications configured in the General Notifications settings are automatically sent to process management assignees when the process management status is changed.
It is not possible to add a message to this type of notifications.

To send notifications with a custom message to assignees, use the Per Record Notifications settings.
However, if notifications to assignees are configured in both the General Notifications settings and the Per Record Notifications settings, duplicate notifications are sent to assignees. To avoid this, configure the settings as follows:

• The General Notifications settings
  Uncheck the Status is changed check box for assignees (or delete the row for assignees)

• The Per Record Notifications settings
  Notification Trigger: Status
  Recipient: Assignee
  Summary: The message to be sent with the notifications

By configuring the Per Record Notifications settings, you will be able to display the custom message with notifications.

By adding notification triggers in the Per Record Notifications settings, you can send different messages for assignees and submitters depending on the status.