In Markup editor used for editing HTML of page, partials, layout, etc, which is in CMS section of the backend, there is a tab beside it called "Code". So, what for is this tab, how is it used, as I read in the documentation example that the template structure which is built up by three sections -- configuration, PHP code, and Twig markup -- can be written in one place?

Based on a template like this:

# This is generated by the UI / Components
title = "Hello world"
==
// This is "Code" tab contents
function onStart()
{
    $this['foo'] = 'bar';
}
==
<!-- This is "Markup" tab contents -->
I am markup. This foo is {{ foo }}.

Try saving a page via the CMS and look at the generated file contents.

I see! Thanks!