Every Product in REX has a Sales Channel and a Source Group attached. This source group is what the order then uses for fulfillment. You can have different source groups for different sales channels. This allows you to alter what outlets are used for stock levels.
If you have 3 in one location and 4 in another you can set up a source group to show you 3, 4 or 7 in stock depending on what you require.
Source groups are maintained via System Settings > Locations/Outlets > Source Groups.
Please refer to Retail Express article Source Groups Overview