When you are ending your inventory session you have the option to specify if it was a partial inventory. A partial inventory session is when you count some, but not all, of your products. For example, some establishments do a full inventory once per month, but do partial inventory of their high volume products once per week to help them know what to order. 

When you end a partial inventory session, the counts you entered will be updated on those items, but any items you did not put in a count for will not have their quantity affected. If an item is out of stock, you should put in 0 for the item in at least one location so it sets the quantity to 0. Any items left blank will not have their quantity adjusted.

Conversely, for a regular full inventory session, Backbar assumes that you counted all items. If an item is left blank, it is assumed that the count is 0. Be sure when you end a full session that you have counted all your items so the quantity does not get set to 0. Before ending you will see a list of items that have not yet been counted so that you can double check.