Custom tabs are used to show the general content for all product detail page, e.g shipping information, Product enquiry guideline, tax related info etc..

By default theme comes with single custom tab, if you want to have more, just need to duplicate the respective scripts blocks.

E.g  consider the following block.

The above script enabled the tab on customization panel as follows.

You can create Shipping info (Or any tab related page) at the page tab in admin panel.

While you add additional tabs, you just need to replace the block numbers and its heading as per your need.