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From Elxis Official Documentation

As of IOS Reservations v3.5 Special Availability is deprecated. You can now set the number of available rooms of each type per season separately and you can also make offline reservations.

On Room Edit page there is a tab called Special Availability. The purpose of this feature is to limit down the number of available rooms for a specific date in order to synchronize IOS Reservations with your offline (or an other online) reservations system. Select your desired date and the number of rooms you wish to limit down for that date, click Apply when ready. To remove an entry click the corresponding delete icon. Notice that entries for past dates are automatically cleaned up from the database. This feature uses AJAX. So you don't have to click Save to save the new entries.

On the same tab you will only notice a link to the Special Availability Calendar. The Special Availability Calendar is like the standard availability calendar except that it also shows you the number of available rooms for each day. You can navigate in time by clicking the forwards and backwards icon links. This calendar is very helpful as it gives you total control over the room's availability. The Special Availability Calendar takes under consideration both guest reserved rooms and the special reserved ones made by you.