Can someone show me filters ?

I’m slowly getting to grips with cs-cart but can’t understand how filters work.

What I would like to do is give my customers the OPTION “See All” or “See only In Stock Items” the default would be see all.

Ideally I would have liked to have added this option to the "View as: " or "Sort by: " but I see the is options to filter but can’t get it to show up for me.

I was wondering if someone could do a simple example, I read the documentation but without examples I’m a bit lost. I’m sure others would benefit from a simple step by step to add a filter to show only in stock item in the whole store.