The availability function allows you or the employees themselves to set them as available for certain time periods.
You can mark people as available for the days when they are able to work - the available days will be displayed on the Schedule Calendar in a light green colour.
To add an availability simply right-click on a person's row and choose 'Mark as available'. Next, a dialogue window will open where you can enter either a single period availability or you can also select multiple days.
Once the availability has been created you can drag and drop shifts onto it as usual.