Employees open Availability in the app and mark per day and service period when they cannot work. A whole period (holiday) is entered as a range. Their preferred number of days per week sits on their profile.
Under Settings you decide until which date employees may fill in availability ('open until') and until which date it is locked ('locked through'). After signup, availability is open eight weeks ahead.
In the roster you see for every shift who is unavailable. You can still schedule them; the app only warns.
Tip for a new team: open the coming month and ask everyone to fill it in before you build the first roster.