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On the Availability tab, in the Quantity group, click Limited, and then select the required function from the drop-down:
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The drop-down list includes three functions:
«+» - add the number of seats specified by the user to the existing allotment.
«-» - decrease the amount of seats by the number specified by the user.
«=» - set the number of seats specified by the user.
2. Click Update.