In addition to being able to directly set your products inventory to a specific value, you can also adjust inventory quantity up or down using Mixtable. Inventory adjustments are useful to ensure that a given product doesn't end up with an incorrect inventory value when multiple users/apps are modifying it. For example, if an order is placed at the same time that Mixtable is setting the inventory to some specific value, you may end up with more or less inventory recorded in Shopify than you do in reality.
To set up a column to adjust the inventory at a location follow these steps. This applies both for products and variants, and so works from a worksheets mapped to products and from worksheets mapped to variants.
Now, to adjust the inventory for a product find the corresponding cell and enter the desired adjustment as a number. A positive number will increase the inventory by that amount. A negative number will decrease the inventory by that amount. Once you've done entering the inventory adjustments for all your products, click on Done Editing and Mixtable will adjust the inventory for you. Once the inventory for a particular product/variant has been successfully adjusted, the cell into which you entered the adjustment will be cleared.