iCalendar (also called iCal) is a digital file format that allows calendar meeting information, such as your day to day tasks or the bookings in your vacation rental. iCal can be shared between different calendar applications.
If you have properties listed in other Sales Channels outside of Rentals United, you can use the Rentals United iCal to import your availability to those Sales Channels. You can also use an external iCal link to import your availability from those Sales Channels to Rentals United.
You can find your RU iCal for each property in RU - Properties - Property - Edit - Step 5 - Availability.
If you are using a PMS, there is no need to use an iCal in Rentals United since availability is synchronised with the PMS.