This file is in the root of the templates folder. It must be modified case is necessary to create one template different of 'default'. They will only be loaded templates that they will be declared in this archive.
As we can see below, it is the representation in XML of a List, where each < string> < /string> he defines a register.

<?xml version="1.0" ?>

<!DOCTYPE list PUBLIC "-//J2EE Spider//DTD Template List 1.0//EN" "http://www.j2eespider.org/dtds/template_list_1_0.dtd">

<list>
    <string>default</string>
</list>

Data of the element <string>
This element defines the name of the folder of template that it goes to be loaded for the SPIDER.
Summarizing: to create new template, it is necessary to create a new folder (with any name) and to declare it in this archive list.xml.
In the example above, 'default' is the name of a folder of templates.
Curiosity:
Opening the folder of templates, we can see 2 folders: 'default' and 'common'. As 'default' is only declared in the archive list.xml, it is only one template.
The folder 'common' is a species of library that contains archives that can be common between several templates, as jar's.