This switch is located at Channel>Channel Settings>Common>Warehouse tab. When this is enabled, selects all available warehouses to do fulfillment for the related channel.
When this switch is disabled, you can see which warehouse is available for the related channel. You can select the specific warehouses from the “Active Warehouses”. When you click the “+” icon in one of the available warehouses, it will be selected for the channel.
NOTE: Please do not include FBA warehouses to channels. StoreAutomator does not manage FBA stocks. If you include FBA warehouses to channel, FBA stocks will show up with your physical stocks in FBM that can cause them to be oversold.