Who is a substitute
A substitute is a person who will take over your responsibilities during your absence. We've added an additional field to the time-off request; it allows you to select your colleague as the substitute. If you are a manager, the substitute will be able to approve or reject requests on behalf of you.
You or your employees will be notified about that when you will turn on the notifications. In order to do that, go to User profile→Notifications:
Other employees will be able to see who is your substitute on the Calendar screen:
If you approve other employees requests, all the requests for approval will be sent to your substitute when you are away.