Use the Select All and Deselect All to quickly select upto 1000 values.
Selecting all will have the same result as not selecting anything as it will select all values including Unknown i.e. all possible records are included in the same way as selecting none. You can wisely use Select All as a shortcut on a standard picklist where you intend to deselect one or more values.
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If you have any questions regarding OSCAR, please contact your account manager.