A global calendar will provide you with an URL to transfer into one main calendar, this will put in all your employees holiday and other leave into your chosen work calendar.
Custom calendars allows you to select certain departments to share calendars.
N.B. The URL that is generated to add to the calendar must not be refreshed, this will cause the sync to stop working.
Once the URL is generated copy it.
If using Google Calendar click here
If using Outlook 365 click here