When an employee isn't available on certain days this can be indicated in the schedule through Availability. The availability can be added via the desktop version and/or the mobile app by either employees and/or supervisors.
When you are adding an availability on the desktop version you have to go to Employees > select an employee > go to Schedule. At the end of every calendar line you will find a green check mark. Through this check mark you can indicate the availability.
When you want to do this in the mobile app you can do that by going to the main menu and click on Availability.
For more information about availability click on the link.