Odoo 17 QWeb Question: Whats behind the concept of <t t-out="0"/>

Question

I am new to Odoo 17. I am developing a module and overriding the sales report template. Here I find the tag

<t t-out="0"/>

that renders the items table. How can I find the template that is rendered when it is called? Where is "0" defined? Thnx!

Edit: I now found the correct template that is rendered when <t t-out="0"/> is called: odoo/addons/sale/report/ir_actions_report_templates.xml

So I was able to copy and use parts of this code. Now I am looking for the template where I can find the section that renders the Optional Products area that is still missing. Thnx!