When you have items in the current order you can choose to add these to a tab by pressing "ADD TO TAB"
If the customer already has a tab, select it from the list and press OK at the "Add To Tab?" prompt.
If you want to add the products to a new tab, press "NEW TAB", type the name for the new tab, and press "OPEN TAB".
Now if you click on that tab in the Summary view, you will see that it has been opened with the items that you added.