If you have so called "contracted rooms" in Booking.com, it means that there are some beds, which should be sold ONLY at Booking.com. 
Therefore, the number of beds which was "contracted" with Booking.com cannot be sold in other means. 

For example - If you have "2 contracted beds" in "6 bed dorm" in Booking.com.

a. You will not be able to change availability on Booking.com to "0" for any date for which  minimum 2 beds were not sold on Booking.com.

b. It means that even if you try to change the availability to "0", because you sold these beds on Hostelworld, Booking.com will answer to you that it is not possible because of contracted rooms. In HostelSystem you will see alerts like this with the content:


"warnings found while updating: "Rooms to Sell were set below the Minimum Contracted rooms for these dates: 2013-08-25, 2013-08-27, 2013-08-26, 2013-08-24, 2013-08-28. The values have been ammended to the Minimum Contracted Rooms"


How to deal with it?


CONTRACTED BEDS IN DORMS


a. Our example - You contracted "2 beds" in "6 bed dorm". 

b. You have Room 1 in HostelSystem with "6 beds". Until now, it was mapped with Hostelworld, Hostelbookers and Booking.com.

c. Now, in order to avoid overbookings, you must create 2 rooms in HostelSystem:


Room 1a with 2 beds - mapped only with Booking.com
Bed 1
Bed 2

Room 1b with 4 beds - mapped with Hostelbookers, Hostelworld and Booking.com
Bed 1
Bed 2
Bed 3
Bed 4

See here: 

Before the change: Example

After the change: Example


This way 2 beds of your ROOM 1 are sold solely to Booking.com and there is no possibility of overbooking.


When all beds are free, HostelSystem will send following availability to portals:

Hostelworld - 4 beds free

Hostelbookers - 4 beds free

Booking.com - 6 beds free


CONTRACTED PRIVATE ROOMS


This is much simpler case. If you have for example 3 x DOUBLE ROOM in your hostel, and one of the rooms was contracted with Booking.com, then simply map only 2 rooms for Hostelworld and Hostelbookers (and others), and all 3 rooms for Booking.com. For example:


Room 1 - Booking.com

Room 2 - Booking.com, Hostelworld, Hostelbookers

Room 3 - Booking.com, Hostelworld, Hostelbookers


This way, room 1 will only host guests from Booking.com.