iCal is a standard calendar file format (.ics) published at a URL. Booking platforms use it to export and import the booked dates of a room or unit, so that a booking on one platform blocks the same dates on the others, without a full channel manager.
Its limits are important: iCal only syncs availability, not rates or restrictions, and it is not real time. Each platform polls the calendar at its own interval, from a few minutes to several hours, which leaves a window in which the same room can be sold twice.
In practice: iCal is a reasonable free option for small properties with a few channels. Check each platform's refresh frequency to know how wide your double-booking window is, and move to a channel manager once volume makes that window too risky.
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