Question
What is the ${URL} in the email notifications used for?
Answer
The line that shows in the source code of your email notifications as <p><a href="${url}" target="_blank">${url}</a></p> is what needs to be there to direct the end user to their training assignment and authenticate them to the platform if your account is set up for ticket-based authentication. It is represented as ${URL} in the Notification editor, where it is the placeholder for the Event URL contact property. Without this contact property in your code, the unique, tokenized link to the training assignment would not be added to your email notifications.
See Platform Contact Properties for additional information.