The last template on the bottom right in the list gallery for each list type is used for temporary list customization. It works in exactly the same way as the other templates, but it is not stored permanently like the other templates. It is overwritten every time you perform a new temporary customization for the current list type.
This works because the templates are not dynamically linked to the lists formatted with them. Changing the list template definition does not change existing lists unless you reformat them with the updated template.
You can use this fact to create and format more different list types than the available number of template slots. To reuse a list type that isn't stored in a template you just need to load it into the temporary list template slot. Then you can apply it to other lists.